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Introduction to Cosmology
Barbara Ryden
Department of Astronomy
The Ohio State University
January 13, 2006
Contents
Preface
v
1 Introduction
1
2 Fundamental Observations
2.1 Dark night sky . . . . . . . . . .
2.2 Isotropy and homogeneity . . . .
2.3 Redshift proportional to distance
2.4 Types of particles . . . . . . . . .
2.5 Cosmic microwave background . .
3 Newton Versus Einstein
3.1 Equivalence principle . .
3.2 Describing curvature . .
3.3 Robertson-Walker metric
3.4 Proper distance . . . . .
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7
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4 Cosmic Dynamics
4.1 Friedmann equation . . . . . . .
4.2 Fluid and acceleration equations
4.3 Equations of state . . . . . . . .
4.4 Learning to love lambda . . . .
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55
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79
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5 Single-Component Universes
5.1 Evolution of energy density
5.2 Curvature only . . . . . . .
5.3 Spatially flat universes . . .
5.4 Matter only . . . . . . . . .
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iii
CONTENTS
5.5
5.6
Radiation only . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Lambda only . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6 Multiple-Component Universes
6.1 Matter + curvature . . . . . .
6.2 Matter + lambda . . . . . . .
6.3 Matter + curvature + lambda
6.4 Radiation + matter . . . . . .
6.5 Benchmark Model . . . . . . .
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7 Measuring Cosmological Parameters
7.1 “A search for two numbers” . . . . .
7.2 Luminosity distance . . . . . . . . . .
7.3 Angular-diameter distance . . . . . .
7.4 Standard candles & H0 . . . . . . . .
7.5 Standard candles & acceleration . . .
8 Dark Matter
8.1 Visible matter . . . . . .
8.2 Dark matter in galaxies .
8.3 Dark matter in clusters .
8.4 Gravitational lensing . .
8.5 What’s the matter? . . .
9 The
9.1
9.2
9.3
9.4
9.5
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Cosmic Microwave Background
Observing the CMB . . . . . . . .
Recombination and decoupling . . .
The physics of recombination . . .
Temperature fluctuations . . . . . .
What causes the fluctuations? . . .
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10 Nucleosynthesis & the Early Universe
10.1 Nuclear physics and cosmology . . . .
10.2 Neutrons and protons . . . . . . . . . .
10.3 Deuterium synthesis . . . . . . . . . .
10.4 Beyond deuterium . . . . . . . . . . .
10.5 Baryon – antibaryon asymmetry . . . .
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101
. 104
. 108
. 112
. 116
. 118
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126
. 126
. 131
. 136
. 141
. 144
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155
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. 159
. 164
. 170
. 175
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179
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. 185
. 189
. 196
. 201
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208
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. 213
. 218
. 222
. 228
iv
CONTENTS
11 Inflation & the Very Early
11.1 The flatness problem . .
11.2 The horizon problem . .
11.3 The monopole problem .
11.4 The inflation solution . .
11.5 The physics of inflation .
12 The
12.1
12.2
12.3
12.4
12.5
Universe
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Formation of Structure
Gravitational instability .
The Jeans length . . . . .
Instability in an expanding
The power spectrum . . .
Hot versus cold . . . . . .
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universe
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233
. 234
. 236
. 238
. 242
. 247
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255
. 258
. 261
. 266
. 272
. 276
Epilogue
284
Annotated Bibliography
286
Preface
This book is based on my lecture notes for an upper-level undergraduate cosmology course at The Ohio State University. The students taking the course
were primarily juniors and seniors majoring in physics and astronomy. In my
lectures, I assumed that my students, having triumphantly survived freshman and sophomore physics, had a basic understanding of electrodynamics,
statistical mechanics, classical dynamics, and quantum physics. As far as
mathematics was concerned, I assumed that, like modern major generals,
they were very good at integral and differential calculus. Readers of this
book are assumed to have a similar background in physics and mathematics. In particular, no prior knowledge of general relativity is assumed; the
(relatively) small amounts of general relativity needed to understand basic
cosmology are introduced as needed.
Unfortunately, the National Bureau of Standards has not gotten around
to establishing a standard notation for cosmological equations. It seems
that every cosmology book has its own notation; this book is no exception.
My main motivation was to make the notation as clear as possible for the
cosmological novice.
I hope that reading this book will inspire students to further explorations
in cosmology. The annotated bibliography at the end of the text provides
a selection of recommended cosmology books, at the popular, intermediate,
and advanced levels.
Many people (too many to name individually) helped in the making of
this book; I thank them all. I owe particular thanks to the students who
took my undergraduate cosmology course at Ohio State University. Their
feedback (including nonverbal feedback such as frowns and snores during
lectures) greatly improved the lecture notes on which this book is based.
Adam Black and Nancy Gee, at Addison Wesley, made possible the great
leap from rough lecture notes to polished book. The reviewers of the text,
v
vi
PREFACE
both anonymous and onymous, pointed out many errors and omissions. I
owe particular thanks to Gerald Newsom, whose careful reading of the entire
manuscript improved it greatly. My greatest debt, however, is to Rick Pogge,
who acted as my computer maven, graphics guru, and sanity check. (He was
also a tireless hunter of creeping fox terrier clones.) As a small sign of my
great gratitude, this book is dedicated to him.
Chapter 1
Introduction
Cosmology is the study of the universe, or cosmos, regarded as a whole. Attempting to cover the study of the entire universe in a single volume may
seem like a megalomaniac’s dream. The universe, after all, is richly textured, with structures on a vast range of scales; planets orbit stars, stars
are collected into galaxies, galaxies are gravitationally bound into clusters,
and even clusters of galaxies are found within larger superclusters. Given
the richness and complexity of the universe, the only way to condense its
history into a single book is by a process of ruthless simplification. For much
of this book, therefore, we will be considering the properties of an idealized, perfectly smooth, model universe. Only near the end of the book will
we consider how relatively small objects, such as galaxies, clusters, and superclusters, are formed as the universe evolves. It is amusing to note, in
this context, that the words “cosmology” and “cosmetology” come from the
same Greek root: the word “kosmos”, meaning harmony or order. Just as
cosmetologists try to make a human face more harmonious by smoothing
over small blemishes such as pimples and wrinkles, cosmologists sometimes
must smooth over small “blemishes” such as galaxies.
A science which regards entire galaxies as being small objects might seem,
at first glance, very remote from the concerns of humanity. Nevertheless, cosmology deals with questions which are fundamental to the human condition.
The questions which vex humanity are given in the title of a painting by Paul
Gauguin (Figure 1.1): “Where do we come from? What are we? Where are
we going?” Cosmology grapples with these questions by describing the past,
explaining the present, and predicting the future of the universe. Cosmologists ask questions such as “What is the universe made of? Is it finite or
1
2
CHAPTER 1. INTRODUCTION
Figure 1.1: Where Do We Come From? What Are We? Where Are We
Going? Paul Gauguin, 1897. [Museum of Fine Arts, Boston]
infinite in spatial extent? Did it have a beginning some time in the past?
Will it come to an end some time in the future?”
Cosmology deals with distances that are very large, objects that are very
big, and timescales that are very long. Cosmologists frequently find that
the standard SI units are not convenient for their purposes: the meter (m)
is awkwardly short, the kilogram (kg) is awkwardly tiny, and the second
(s) is awkwardly brief. Fortunately, we can adopt the units which have been
developed by astronomers for dealing with large distances, masses, and times.
One distance unit used by astronomers is the astronomical unit (AU),
equal to the mean distance between the Earth and Sun; in metric units,
1 AU = 1.5 × 1011 m. Although the astronomical unit is a useful length scale
within the Solar System, it is small compared to the distances between stars.
To measure interstellar distances, it is useful to use the parsec (pc), equal
to the distance at which 1 AU subtends an angle of 1 arcsecond; in metric
units, 1 pc = 3.1 × 1016 m. For example, we are at a distance of 1.3 pc from
Proxima Centauri (the Sun’s nearest neighbor among the stars) and 8500 pc
from the center of our Galaxy. Although the parsec is a useful length scale
within our Galaxy, it is small compared to the distances between galaxies.
To measure intergalactic distances, it is useful to use the megaparsec (Mpc),
equal to 106 pc, or 3.1 × 1022 m. For example, we are at a distance of 0.7 Mpc
from M31 (otherwise known as the Andromeda galaxy) and 15 Mpc from the
Virgo cluster (the nearest big cluster of galaxies).
The standard unit of mass used by astronomers is the solar mass (M¯ );
in metric units, the Sun’s mass is 1 M¯ = 2.0 × 1030 kg. The total mass
3
of our Galaxy is not known as accurately as the mass of the Sun; in round
numbers, though, it is Mgal ≈ 1012 M¯ . The Sun, incidentally, also provides
the standard unit of power used in astronomy. The Sun’s luminosity (that
is, the rate at which it radiates away energy in the form of light) is 1 L¯ =
3.8 × 1026 watts. The total luminosity of our Galaxy is Lgal = 3.6 × 1010 L¯ .
For times much longer than a second, astronomers use the year (yr),
defined as the time it takes the Earth to go once around the Sun. One
year is approximately equal to 3.2 × 107 s. In cosmological context, a year is
frequently an inconveniently short period of time, so cosmologists frequently
use gigayears (Gyr), equal to 109 yr, or 3.2 × 1016 s. For example, the age of
the Earth is more conveniently written as 4.6 Gyr than as 1.5 × 1017 s.
In addition to dealing with very large things, cosmology also deals with
very small things. Early in its history, as we shall see, the universe was
very hot and dense, and some interesting particle physics phenomena were
occurring. Consequently, particle physicists have plunged into cosmology,
introducing some terminology and units of their own. For instance, particle
physicists tend to measure energy units in electron volts (eV) instead of joules
(J). The conversion factor between electron volts and joules is 1 eV = 1.6 ×
10−19 J. The rest energy of an electron, for instance, is me c2 = 511,000 eV =
0.511 MeV, and the rest energy of a proton is mP c2 = 938.3 MeV.
When you stop to think of it, you realize that the units of meters,
megaparsecs, kilograms, solar masses, seconds, and gigayears could only be
devised by ten-fingered Earthlings obsessed with the properties of water.
An eighteen-tentacled silicon-based lifeform from a planet orbiting Betelgeuse would devise a different set of units. A more universal, less culturally biased, system of units is the Planck system, based on the universal
constants G, c, and h̄. Combining the Newtonian gravitational constant,
G = 6.7 × 10−11 m3 kg−1 s−2 , the speed of light, c = 3.0 × 108 m s−1 , and the
reduced Planck constant, h̄ = h/(2π) = 1.1 × 10−34 J s = 6.6 × 10−16 eV s,
yields a unique length scale, known as the Planck length:
`P ≡
Ã
Gh̄
c3
!1/2
= 1.6 × 10−35 m .
(1.1)
4
CHAPTER 1. INTRODUCTION
The same constants can be combined to yield the Planck mass,1
MP ≡
h̄c
G
!1/2
= 2.2 × 10−8 kg ,
(1.2)
Gh̄
c5
!1/2
= 5.4 × 10−44 s .
(1.3)
Ã
and the Planck time,
tP ≡
Ã
Using Einstein’s relation between mass and energy, we can also define the
Planck energy,
EP = MP c2 = 2.0 × 109 J = 1.2 × 1028 eV .
(1.4)
By bringing the Boltzmann constant, k = 8.6 × 10−5 eV K−1 , into the act,
we can also define the Planck temperature,
TP = EP /k = 1.4 × 1032 K .
(1.5)
When distance, mass, time, and temperature are measured in the appropriate
Planck units, then c = k = h̄ = G = 1. This is convenient for individuals who
have difficulty in remembering the numerical values of physical constants.
However, using Planck units can have potentially confusing side effects. For
instance, many cosmology texts, after noting that c = k = h̄ = G = 1
when Planck units are used, then proceed to omit c, k, h̄, and/or G from all
equations. For instance, Einstein’s celebrated equation, E = mc2 , becomes
E = m. The blatant dimensional incorrectness of such an equation is jarring,
but it simply means that the rest energy of an object, measured in units of
the Planck energy, is equal to its mass, measured in units of the Planck mass.
In this book, however, I will retain all factors of c, k, h̄, and G, for the sake
of clarity.
In this book, we will be dealing with distances ranging from the Planck
length to 104 Mpc or so, a span of some 61 orders of magnitude. Dealing with
such a wide range of length scales requires a stretch of the imagination, to
be sure. However, cosmologists are not permitted to let their imaginations
run totally unfettered. Cosmology, I emphasize strongly, is ultimately based
1
The Planck mass is roughly equal to the mass of a grain of sand a quarter of a
millimeter across.
5
Figure 1.2: The ancient Egyptian view of the cosmos: the sky goddess Nut,
supported by the air god Shu, arches over the earth god Geb (from the
c
Greenfield Papyrus, ca. 1025 BC). [°Copyright
The British Museum]
on observation of the universe around us. Even in ancient times, cosmology
was based on observations; unfortunately, those observations were frequently
imperfect and incomplete. Ancient Egyptians, for instance, looked at the
desert plains stretching away from the Nile valley and the blue sky overhead.
Based on their observations, they developed a model of the universe in which
a flat Earth (symbolized by the earth god Geb in Figure 1.2) was covered by a
solid dome (symbolized by the sky goddess Nut). Greek cosmology was based
on more precise and sophisticated observations. Ancient Greek astronomers
deduced, from their observations, that the Earth and Moon are spherical,
that the Sun is much farther from the Earth than the Moon is, and that
the distance from the Earth to the stars is much greater than the Earth’s
diameter. Based on this knowledge, Greek cosmologists devised a “twosphere” model of the universe, in which the spherical Earth is surrounded
by a much larger celestial sphere, a spherical shell to which the stars are
attached. Between the Earth and the celestial sphere, in this model, the
Sun, Moon, and planets move on their complicated apparatus of epicycles
and deferents.
Although cosmology is ultimately based on observation, sometimes obser-
6
CHAPTER 1. INTRODUCTION
vations temporarily lag behind theory. During periods when data are lacking,
cosmologists may adopt a new model for aesthetic or philosophical reasons.
For instance, when Copernicus proposed a new Sun-centered model of the
universe, to replace the Earth-centered two-sphere model of the Greeks, he
didn’t base his model on new observational discoveries. Rather, he believed
that putting the Earth in motion around the Sun resulted in a conceptually
simpler, more appealing model of the universe. Direct observational evidence
didn’t reveal that the Earth revolves around the Sun, rather than vice versa,
until the discovery of the aberration of starlight in the year 1728, nearly two
centuries after the death of Copernicus. Foucault didn’t demonstrate the rotation of the Earth, another prediction of the Copernican model, until 1851,
over three centuries after the death of Copernicus. However, although observations sometimes lag behind theory in this way, every cosmological model
that isn’t eventually supported by observational evidence must remain pure
speculation.
The current standard model for the universe is the “Hot Big Bang” model,
which states that the universe has expanded from an initially hot and dense
state to its current relatively cool and tenuous state, and that the expansion
is still going on today. To see why cosmologists have embraced the Hot Big
Bang model, let us turn, in the next chapter, to the fundamental observations
on which modern cosmology is based.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Cox (2000): Accurate values of physical and astronomical constants
Harrison (2000), ch. 1 – 4: A history of early (pre-Einstein) cosmology
Chapter 2
Fundamental Observations
Some of the observations on which modern cosmology is based are highly
complex, requiring elaborate apparatus and sophisticated data analysis. However, other observations are surprisingly simple. Let’s start with an observation which is deceptive in its extreme simplicity.
2.1
The night sky is dark
Step outside on a clear, moonless night, far from city lights, and look upward.
You will see a dark sky, with roughly two thousand stars scattered across
it. The fact that the night sky is dark at visible wavelengths, instead of
being uniformly bright with starlight, is known as Olbers’ Paradox, after the
astronomer Heinrich Olbers, who wrote a scientific paper on the subject in
the year 1826. As it happens, Olbers was not the first person to think about
Olbers’ Paradox. As early as 1576, Thomas Digges mentioned how strange
it is that the night sky is dark, with only a few pinpoints of light to mark
the location of stars.1
Why should it be paradoxical that the night sky is dark? Most of us
simply take for granted the fact that daytime is bright and nighttime is dark.
The darkness of the night sky certainly posed no problems to the ancient
Egyptians or Greeks, to whom stars were points of light stuck to a dome or
sphere. However, the cosmological model of Copernicus required that the
1
The name “Olbers’ Paradox” is thus a prime example of what historians of science
jokingly call the law of misonomy: nothing is ever named after the person who really
discovers it.
7
8
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
r
dr
Figure 2.1: A star-filled spherical shell, of radius r and thickness dr, centered
on the Earth.
distance to stars be very much larger than an astronomical unit; otherwise,
the parallax of the stars, as the Earth goes around on its orbit, would be
large enough to see with the naked eye. Moreover, since the Copernican
system no longer requires that the stars be attached to a rotating celestial
sphere, the stars can be at different distances from the Sun. These liberating
realizations led Thomas Digges, and other post-Copernican astronomers, to
embrace a model in which stars are large glowing spheres, like the Sun,
scattered throughout infinite space.
Let’s compute how bright we expect the night sky to be in an infinite
universe. Let n be the average number density of stars in the universe, and
let L be the average stellar luminosity. The flux received here at Earth from
a star of luminosity L at a distance r is given by an inverse square law:
f (r) =
L
.
4πr2
(2.1)
Now consider a thin spherical shell of stars, with radius r and thickness dr,
centered on the Earth (Figure 2.1). The intensity of radiation from the shell
of stars (that is, the power per unit area per steradian of the sky) will be
dJ(r) =
L
nL
2
·
n
·
r
dr
=
dr .
4πr2
4π
(2.2)
9
2.1. DARK NIGHT SKY
The total intensity of starlight from a shell thus depends only on its thickness,
not on its distance from us. We can compute the total intensity of starlight
from all the stars in the universe by integrating over shells of all radii:
J=
Z
∞
r=0
dJ =
nL Z ∞
dr = ∞ .
4π 0
(2.3)
Thus, I have demonstrated that the night sky is infinitely bright.
This is utter nonsense.
Therefore, one (or more) of the assumptions that went into the above
analysis of the sky brightness must be wrong. Let’s scrutinize some of the
assumptions. One assumption that I made is that we have an unobstructed
line of sight to every star in the universe. This is not true. In fact, since
stars have a finite angular size as seen from Earth, nearby stars will hide
more distant stars from our view. Nevertheless, in an infinite distribution of
stars, every line of sight should end at the surface of a star; this would imply
a surface brightness for the sky equal to the surface brightness of a typical
star. This is an improvement on an infinitely bright sky, but is still distinctly
different from the dark sky which we actually see. Heinrich Olbers himself
tried to resolve Olbers’ Paradox by proposing that distant stars are hidden
from view by interstellar matter which absorbs starlight. This resolution
will not work, because the interstellar matter will be heated by starlight
until it has the same temperature as the surface of a star. At that point,
the interstellar matter emits as much light as it absorbs, and is glowing as
brightly as the stars themselves.
A second assumption I made is that the number density n and mean
luminosity L of stars are constant throughout the universe; more accurately,
the assumption made in equation (2.3) is that the product nL is constant as
a function of r. This might not be true. Distant stars might be less luminous
or less numerous than nearby stars. If we are in a clump of stars of finite size,
then the absence of stars at large distances will keep the night sky from being
bright. Similarly, if distant stars are sufficiently low in luminosity compared
to nearby stars, they won’t contribute significantly to the sky brightness. In
order for the integrated intensity in equation (2.3) to be finite, the product
nL must fall off more rapidly than nL ∝ 1/r as r → ∞.
A third assumption is that the universe is infinitely large. This might
not be true. If the universe only extends to a maximum distance rmax from
us, then the total intensity of starlight we see in the night sky will be J ∼
10
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
nLrmax /(4π). Note that this result will also be found if the universe is infinite
in space, but is devoid of stars beyond a distance rmax .
A fourth assumption, slightly more subtle than the previous ones, is that
the universe is infinitely old. This might not be true. Since the speed of
light is finite, when we look farther out in space, we are looking farther out
in time. Thus, we see the Sun as it was 8.3 minutes ago, Proxima Centauri
as it was 4 years ago, and M31 as it was 2 million years ago. If the universe
has a finite age t0 , the intensity of starlight we see at night will be at most
J ∼ nLct0 /(4π). Note that this result will also be found if the universe is
infinitely old, but has only contained stars for a finite time t0 .
A fifth assumption is that the flux of light from a distant source is given
by the inverse square law of equation (2.1). This might not be true. The
assumption that f ∝ 1/r 2 would have seemed totally innocuous to Olbers
and other nineteenth century astronomers; after all, the inverse square law
follows directly from Euclid’s laws of geometry. However, in the twentieth
century, Albert Einstein, that great questioner of assumptions, demonstrated
that the universe might not obey the laws of Euclidean geometry. In addition,
the inverse square law assumes that the source of light is stationary relative
to the observer. If the universe is systematically expanding or contracting,
then the light from distant sources will be redshifted to lower photon energies
or blueshifted to higher photon energies.
Thus, the infinitely large, eternally old, Euclidean universe which Thomas
Digges and his successors pictured simply does not hold up to scrutiny. This
is a textbook, not a suspense novel, so I’ll tell you right now: the primary
resolution to Olbers’ Paradox comes from the fact that the universe has a
finite age. The stars beyond some finite distance, called the horizon distance,
are invisible to us because their light hasn’t had time to reach us yet. A
particularly amusing bit of cosmological trivia is that the first person to hint
at the correct resolution of Olbers’ Paradox was Edgar Allen Poe.2 In his
essay “Eureka: A Prose Poem”, completed in the year 1848, Poe wrote,
“Were the succession of stars endless, then the background of the sky would
present us an [sic] uniform density . . . since there could be absolutely no
point, in all that background, at which would not exist a star. The only
mode, therefore, in which, under such a state of affairs, we could comprehend
the voids which our telescopes find in innumerable directions, would be by
supposing the distance of the invisible background so immense that no ray
2
That’s right, the “Nevermore” guy.
2.2. ISOTROPY AND HOMOGENEITY
11
from it has yet been able to reach us at all.”
2.2
On large scales, the universe is isotropic
and homogeneous
What does it mean to state that the universe is isotropic and homogeneous?
Saying that the universe is isotropic means that there are no preferred directions in the universe; it looks the same no matter which way you point your
telescope. Saying that the universe is homogeneous means that there are no
preferred locations in the universe; it looks the same no matter where you set
up your telescope. Note the very important qualifier: the universe is isotropic
and homogeneous on large scales. In this context, “large scales” means that
the universe is only isotropic and homogeneous on scales of roughly 100 Mpc
or more.
The isotropy of the universe is not immediately obvious. In fact, on small
scales, the universe is blatantly anisotropic. Consider, for example, a sphere
3 meters in diameter, centered on your navel (Figure 2.2a). Within this
sphere, there is a preferred direction; it is the direction commonly referred
to as “down”. It is easy to determine the vector pointing down. Just let go
of a small dense object. The object doesn’t hover in midair, and it doesn’t
move in a random direction; it falls down, toward the center of the Earth.
On significantly larger scales, the universe is still anisotropic. Consider,
for example, a sphere 3 AU in diameter, centered on your navel (Figure 2.2b).
Within this sphere, there is a preferred direction; it is the direction pointing
toward the Sun, which is by far the most massive and most luminous object
within the sphere. It is easy to determine the vector pointing toward the
Sun. Just step outside on a sunny day, and point to that really bright disk
of light up in the sky.
On still large scales, the universe is still anisotropic. Consider, for example, a sphere 3 Mpc in diameter, centered on your navel (Figure 2.2c).
This sphere contains the Local Group of galaxies, a small cluster of some 40
galaxies. By far the most massive and most luminous galaxies in the Local
Group are our own Galaxy and M31, which together contribute about 86% of
the total luminosity within the 3 Mpc sphere. Thus, within this sphere, our
Galaxy and M31 define a preferred direction. It is fairly easy to determine
the vector pointing from our Galaxy to M31; just step outside on a clear
12
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
Figure 2.2: (a) A sphere 3 meters in diameter, centered on your navel. (b)
A sphere 3 AU in diameter, centered on your navel. (c) A sphere 3 Mpc
in diameter, centered on your navel. (d) A sphere 200 Mpc in diameter,
centered on your navel. Shown is the number density of galaxies smoothed
with a Gaussian of width 17 Mpc. The heavy contour is drawn at the mean
density; darker regions represent higher density, lighter regions represent
lower density (from Dekel et al. 1999, ApJ, 522, 1, fig. 2)
2.2. ISOTROPY AND HOMOGENEITY
13
night when the constellation Andromeda is above the horizon, and point to
the fuzzy oval in the middle of the constellation.
It isn’t until you get to considerably larger scales that the universe can be
considered as isotropic. Consider a sphere 200 Mpc in diameter, centered on
your navel. Figure 2.2d shows a slice through such a sphere, with superclusters of galaxies indicated as dark patches. The Perseus-Pisces supercluster is
on the right, the Hydra-Centaurus supercluster is on the left, and the edge of
the Coma supercluster is just visible at the top of Figure 2.2d. Superclusters
are typically ∼ 100 Mpc along their longest dimensions, and are separated
by voids (low density regions) which are typically ∼ 100 Mpc across. These
are the largest structures in the universe, it seems; surveys of the universe
on still larger scales don’t find “superduperclusters”.
On small scales, the universe is obviously inhomogeneous, or lumpy, in
addition to being anisotropic. For instance, a sphere 3 meters in diameter,
centered on your navel, will have an average density of ∼ 100 kg m−3 , in
round numbers. However, the average density of the universe as a whole is
ρ0 ∼ 3 × 10−27 kg m−3 . Thus, on a scale d ∼ 3 m, the patch of the universe
surrounding you is more than 28 orders of magnitude denser than average.
On significantly larger scales, the universe is still inhomogeneous. A
sphere 3 AU in diameter, centered on your navel, has an average density
of 4 × 10−5 kg m−3 ; that’s 22 orders of magnitude denser than the average for
the universe.
On still larger scales, the universe is still inhomogeneous. A sphere 3
Mpc in diameter, centered on your navel, will have an average density of
∼ 3 × 10−26 kg m−3 , still an order of magnitude denser than the universe as a
whole. It’s only when you contemplate a sphere ∼ 100 Mpc in diameter that
a sphere centered on your navel is not overdense compared to the universe
as a whole.
Note that homogeneity does not imply isotropy. A sheet of paper printed
with stripes (Figure 2.3a) is homogeneous on scales larger than the stripe
width, but it is not isotropic. The direction of the stripes provides a preferred
direction by which you can orient yourself. Note also that isotropy around
a single point does not imply homogeneity. A sheet of paper printed with a
bullseye (Figure 2.3b) is isotropic around the center of the bullseye, but is it
not homogeneous. The rings of the bullseye look different far from the center
than they do close to the center. You can tell where you are relative to the
center by measuring the radius of curvature of the nearest ring.
In general, then, saying that something is homogeneous is quite different
14
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
Figure 2.3: (a) A pattern which is anisotropic, but which is homogeneous on
scales larger than the stripe width. (b) A pattern which is isotropic about
the origin, but which is inhomogeneous.
from saying it is isotropic. However, modern cosmologists have adopted
the cosmological principle, which states “There is nothing special about our
location in the universe.” The cosmological principle holds true only on large
scales (of 100 Mpc or more). On smaller scales, your navel obviously is in
a special location. Most spheres 3 meters across don’t contain a sentient
being; most sphere 3 AU across don’t contain a star; most spheres 3 Mpc
across don’t contain a pair of bright galaxies. However, most spheres over 100
Mpc across do contain roughly the same pattern of superclusters and voids,
statistically speaking. The universe, on scales of 100 Mpc or more, appears
to be isotropic around us. Isotropy around any point in the universe, such
as your navel, combined with the cosmological principle, implies isotropy
around every point in the universe; and isotropy around every point in the
universe does imply homogeneity.
The cosmological principle has the alternate name of the “Copernican
principle” as a tribute to Copernicus, who pointed out that the Earth is not
the center of the universe. Later cosmologists also pointed out the Sun is not
the center, that our Galaxy is not the center, and that the Local Group is
not the center. In fact, there is no center to the universe.
2.3. REDSHIFT PROPORTIONAL TO DISTANCE
2.3
15
Galaxies show a redshift proportional to
their distance
When we look at a galaxy at visible wavelengths, we are primarily detecting
the light from the stars which the galaxy contains. Thus, when we take
a galaxy’s spectrum at visible wavelengths, it typically contains absorption
lines created in the stars’ relatively cool upper atmospheres.3 Suppose we
consider a particular absorption line whose wavelength, as measured in a
laboratory here on Earth, is λem . The wavelength we measure for the same
absorption line in a distant galaxy’s spectrum, λob , will not, in general, be
the same. We say that the galaxy has a redshift z, given by the formula
z≡
λob − λem
.
λem
(2.4)
Strictly speaking, when z < 0, this quantity is called a blueshift, rather than
a redshift. However, the vast majority of galaxies have z > 0.
The fact that the light from galaxies is generally redshifted to longer
wavelengths, rather than blueshifted to shorter wavelengths, was not known
until the twentieth century. In 1912, Vesto Slipher, at the Lowell Observatory,
measured the shift in wavelength of the light from M31; this galaxy, as it
turns out, is one of the few which exhibits a blueshift. By 1925, Slipher
had measured the shifts in the spectral lines for approximately 40 galaxies,
finding that they were nearly all redshifted; the exceptions were all nearby
galaxies within the Local Group.
By 1929, enough galaxy redshifts had been measured for the cosmologist
Edwin Hubble to make a study of whether a galaxy’s redshift depends on
its distance from us. Although measuring a galaxy’s redshift is relatively
easy, and can be done with high precision, measuring its distance is difficult.
Hubble knew z for nearly 50 galaxies, but had estimated distances for only
20 of them. Nevertheless, from a plot of redshift (z) versus distance (r),
reproduced in Figure 2.4, he found the famous linear relation now known as
Hubble’s Law:
H0
z=
r,
(2.5)
c
where H0 is a constant (now called the Hubble constant). Hubble interpreted
the observed redshift of galaxies as being a Doppler shift due to their radial
3
Galaxies containing active galactic nuclei will also show emission lines from the hot
gas in their nuclei.
16
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
Figure 2.4: Edwin Hubble’s original plot of the relation between redshift
(vertical axis) and distance (horizontal axis). Note that the vertical axis
actually plots cz rather than z – and that the units are accidentally written
as km rather than km/s. (from Hubble 1929, Proc. Nat. Acad. Sci., 15,
168)
2.3. REDSHIFT PROPORTIONAL TO DISTANCE
17
Figure 2.5: A more modern version of Hubble’s plot, showing cz versus
distance. In this case, the galaxy distances have been determined using
Cepheid variable stars as standard candles, as described in Chapter 6. (from
Freedman, et al. 2001, ApJ, 553, 47)
velocity away from Earth. Since the values of z in Hubble’s analysis were all
small (z < 0.04), he was able to use the classical, nonrelativistic relation for
the Doppler shift, z = v/c, where v is the radial velocity of the light source
(in this case, a galaxy). Interpreting the redshifts as Doppler shifts, Hubble’s
law takes the form
v = H0 r .
(2.6)
Since the Hubble constant H0 can be found by dividing velocity by distance,
it is customarily written in the rather baroque units of km s−1 Mpc−1 . When
Hubble first discovered Hubble’s Law, he thought that the numerical value of
the Hubble constant was H0 = 500 km s−1 Mpc−1 (see Figure 2.4). However,
it turned out that Hubble was severely underestimating the distances to
galaxies.
Figure 2.5 shows a more recent determination of the Hubble constant
from nearby galaxies, using data obtained by (appropriately enough) the
18
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
2
r12
1
r23
3
r31
Figure 2.6: A triangle defined by three galaxies in a uniformly expanding
universe.
Hubble Space Telescope. The best current estimate of the Hubble constant,
combining the results of different research groups, is
H0 = 70 ± 7 km s−1 Mpc−1 .
(2.7)
This is the value for the Hubble constant that I will use in the remainder of
this book.
Cosmological innocents sometimes exclaim, when first encountering Hubble’s Law, “Surely it must be a violation of the cosmological principle to
have all those distant galaxies moving away from us! It looks as if we are
at a special location in the universe – the point away from which all other
galaxies are fleeing.” In fact, what we see here in our Galaxy is exactly what
you would expect to see in a universe which is undergoing homogeneous and
isotropic expansion. We see distant galaxies moving away from us; but observers in any other galaxy would also see distant galaxies moving away from
them.
To see on a more mathematical level what we mean by homogeneous,
isotropic expansion, consider three galaxies at positions ~r1 , ~r2 , and ~r3 . They
define a triangle (Figure 2.6) with sides of length
r12 ≡ |~r1 − ~r2 |
r23 ≡ |~r2 − ~r3 |
r31 ≡ |~r3 − ~r1 | .
(2.8)
(2.9)
(2.10)
2.3. REDSHIFT PROPORTIONAL TO DISTANCE
19
Homogeneous and uniform expansion means that the shape of the triangle
is preserved as the galaxies move away from each other. Maintaining the
correct relative lengths for the sides of the triangle requires an expansion law
of the form
r12 (t) = a(t)r12 (t0 )
r23 (t) = a(t)r23 (t0 )
r31 (t) = a(t)r31 (t0 ) .
(2.11)
(2.12)
(2.13)
Here the function a(t) is a scale factor which is equal to one at the present
moment (t = t0 ) and which is totally independent of location or direction.
The scale factor a(t) tells us how the expansion (or possibly contraction) of
the universe depends on time. At any time t, an observer in galaxy 1 will
see the other galaxies receding with a speed
dr12
= ȧr12 (t0 ) =
dt
dr31
v31 (t) =
= ȧr31 (t0 ) =
dt
v12 (t) =
ȧ
r12 (t)
a
ȧ
r31 (t) .
a
(2.14)
(2.15)
You can easily demonstrate that an observer in galaxy 2 or galaxy 3 will
find the same linear relation between observed recession speed and distance,
with ȧ/a playing the role of the Hubble constant. Since this argument can
be applied to any trio of galaxies, it implies that in any universe where the
distribution of galaxies is undergoing homogeneous, isotropic expansion, the
velocity – distance relation takes the linear form v = Hr, with H = ȧ/a.
If galaxies are currently moving away from each other, this implies they
were closer together in the past. Consider a pair of galaxies currently separated by a distance r, with a velocity v = H0 r relative to each other. If
there are no forces acting to accelerate or decelerate their relative motion,
then their velocity is constant, and the time that has elapsed since they were
in contact is
r
r
t0 = =
= H0−1 ,
(2.16)
v
H0 r
independent of the current separation r. The time H0−1 is referred to as the
Hubble time. For H0 = 70 ± 7 km s−1 Mpc−1 , the Hubble time is H0−1 =
14.0 ± 1.4 Gyr. If the relative velocities of galaxies have been constant in the
past, then one Hubble time ago, all the galaxies in the universe were crammed
together into a small volume. Thus, the observation of galactic redshifts lead
20
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
naturally to a Big Bang model for the evolution of the universe. A Big Bang
model may be broadly defined as a model in which the universe expands from
an initially highly dense state to its current low-density state.
The Hubble time of ∼ 14 Gyr is comparable to the ages computed for
the oldest known stars in the universe. This rough equivalence is reassuring.
However, the age of the universe – that is, the time elapsed since its original
highly dense state – is not necessarily exactly equal to the Hubble time.
We know that gravity exists, and that galaxies contain matter. If gravity
working on matter is the only force at work on large scales, then the attractive
force of gravity will act to slow the expansion. In this case, the universe
was expanding more rapidly in the past than it is now, and the universe is
younger than H0−1 . On the other hand, if the energy density of the universe
is dominated by a cosmological constant (an entity which we’ll examine in
more detail in Chapter 4), then the dominant gravitational force is repulsive,
and the universe may be older than H0−1 .
Just as the Hubble time provides a natural time scale for our universe,
the Hubble distance, c/H0 = 4300 ± 400 Mpc, provides a natural distance
scale. Just as the age of the universe is roughly equal to H0−1 in most Big
Bang models, with the exact value depending on the expansion history of the
universe, so the horizon distance (the greatest distance a photon can travel
during the age of the universe) is roughly equal to c/H0 , with the exact value,
again, depending on the expansion history. (Later chapters will deal with
computing the exact values of the age and horizon size of our universe.)
Note how Hubble’s Law ties in with Olbers’ Paradox. If the universe is
of finite age, t0 ∼ H0−1 , then the night sky can be dark, even if the universe
is infinitely large, because light from distant galaxies has not yet had time
to reach us. Galaxy surveys tell us that the luminosity density of galaxies in
the local universe is
nL ≈ 2 × 108 L¯ Mpc−3 .
(2.17)
By terrestrial standards, the universe is not a well-lit place; this luminosity
density is equivalent to a single 40 watt light bulb within a sphere 1 AU in
radius. If the horizon distance is dhor ∼ c/H0 , then the total flux of light we
receive from all the stars from all the galaxies within the horizon will be
Fgal
c
dr ∼ nL
= 4πJgal ≈ nL
H0
0
−2
11
∼ 9 × 10 L¯ Mpc ∼ 2 × 10−11 L¯ AU−2 .
Z
rH
µ
¶
(2.18)
2.3. REDSHIFT PROPORTIONAL TO DISTANCE
21
By the cosmological principle, this is the total flux of starlight you would
expect at any randomly located spot in the universe. Comparing this to the
flux we receive from the Sun,
Fsun =
1 L¯
≈ 0.08 L¯ AU−2 ,
4π AU2
(2.19)
we find that Fgal /Fsun ∼ 3 × 10−10 . Thus, the total flux of starlight at a
randomly selected location in the universe is less than a billionth the flux of
light we receive from the Sun here on Earth. For the entire universe to be as
well-lit as the Earth, it would have to be over a billion times older than it is;
and you’d have to keep the stars shining during all that time.
Hubble’s Law occurs naturally in a Big Bang model for the universe, in
which homogeneous and isotropic expansion causes the density of the universe
to decrease steadily from its initial high value. In a Big Bang model, the
properties of the universe evolve with time; the average density decreases, the
mean distance between galaxies increases, and so forth. However, Hubble’s
Law can also be explained by a Steady State model. The Steady State model
was first proposed in the 1940’s by Hermann Bondi, Thomas Gold, and
Fred Hoyle, who were proponents of the perfect cosmological principle, which
states that not only are there no privileged locations in space, there are no
privileged moments in time. Thus, a Steady State universe is one in which
the global properties of the universe, such as the mean density ρ0 and the
Hubble constant H0 , remain constant with time.
In a Steady State universe, the velocity – distance relation
dr
= H0 r
dt
(2.20)
can be easily integrated, since H0 is constant with time, to yield an exponential law:
r(t) ∝ eH0 t .
(2.21)
Note that r → 0 only in the limit t → −∞; a Steady State universe is
infinitely old. If there existed an instant in time at which the universe started
expanding (as in a Big Bang model), that would be a special moment, in
violation of the assumed “perfect cosmological principle”. The volume of a
spherical region of space, in a Steady State model, increases exponentially
with time:
4π 3
V =
r ∝ e3H0 t .
(2.22)
3
22
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
However, if the universe is in a steady state, the density of the sphere must
remain constant. To have a constant density of matter within a growing
volume, matter must be continuously created at a rate
Ṁss = ρ0 V̇ = ρ0 3H0 V .
(2.23)
If our own universe, with matter density ρ0 ∼ 3 × 10−27 kg m−3 , happened to
be a Steady State universe, then matter would have to be created at a rate
Ṁss
= 3H0 ρ0 ∼ 6 × 10−28 kg m−3 Gyr−1 .
(2.24)
V
This corresponds to creating roughly one hydrogen atom per cubic kilometer
per year.
During the 1950s and 1960s, the Big Bang and Steady State models battled for supremacy. Critics of the Steady State model pointed out that the
continuous creation of matter violates mass-energy conservation. Supporters
of the Steady State model pointed out that the continuous creation of matter
is no more absurd that the instantaneous creation of the entire universe in a
single “Big Bang” billions of years ago.4 The Steady State model finally fell
out of favor when observational evidence increasingly indicated that the perfect cosmological principle is not true. The properties of the universe do, in
fact, change with time. The discovery of the Cosmic Microwave Background,
discussed below in section 2.5, is commonly regarded as the observation which
decisively tipped the scales in favor of the Big Bang model.
2.4
The universe contains different types of
particles
It doesn’t take a brilliant observer to confirm that the universe contains a
large variety of different things: ships, shoes, sealing wax, cabbages, kings,
galaxies, and what have you. From a cosmologist’s viewpoint, though, cabbages and kings are nearly indistinguishable – the main difference between
them is that the mean mass per king is greater than the mean mass per cabbage. From a cosmological viewpoint, the most significant difference between
the different components of the universe is that they are made of different
elementary particles. The properties of the most cosmologically important
4
The name “Big Bang” was actually coined by Fred Hoyle, a supporter of the Steady
State model.
2.4. TYPES OF PARTICLES
23
Table 2.1: Particle Properties
particle
symbol rest energy (MeV) charge
proton
p
938.3
+1
neutron
n
939.6
0
electron
e−
0.511
-1
neutrino
νe ,νµ ,ντ
?
0
photon
γ
0
0
dark matter
?
?
0
particles are summarized in Table 2.1.
The material objects which surround us during our everyday life are made
of protons, neutrons, and electrons.5 Protons and neutrons are both examples
of baryons, where a baryon is defined as a particle made of three quarks. A
proton (p) contains two “up” quarks, each with an electrical charge of +2/3,
and a “down” quark, with charge −1/3. A neutron (n) contains one “up”
quark and two “down” quarks. Thus a proton has a net positive charge
of +1, while a neutron is electrically neutral. Protons and neutrons also
differ in their mass – or equivalently, in their rest energies. The proton mass
is mp c2 = 938.3 MeV, while the neutron mass is mn c2 = 939.6 MeV, about
0.1% greater. Free neutrons are unstable, decaying into protons with a decay
time of τn = 940 s, about a quarter of an hour. By contrast, experiments
have put a lower limit on the decay time of the proton which is very much
greater than the Hubble time. Neutrons can be preserved against decay by
binding them into an atomic nucleus with one or more protons.
Electrons (e− ) are examples of leptons, a class of elementary particles
which are not made of quarks. The mass of an electron is much smaller
than that of a neutron or proton; the rest energy of an electron is me c2 =
0.511 MeV. An electron has an electric charge equal in magnitude to that of
a proton, but opposite in sign. On large scales, the universe is electrically
neutral; the number of electrons is equal to the number of protons. Since protons outmass electrons by a factor of 1836 to 1, the mass density of electrons
is only a small perturbation to the mass density of protons and neutrons.
For this reason, the component of the universe made up of ions, atoms, and
molecules is generally referred to as baryonic matter, since only the baryons
(protons and neutrons) contribute significantly to the mass density. Protons
5
For that matter, we ourselves are made of protons, neutrons, and electrons.
24
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
and neutrons are 800-pound gorillas; electrons are only 7-ounce bushbabies.
About three-fourths of the baryonic matter in the universe is currently in
the form of ordinary hydrogen, the simplest of all elements. In addition, when
we look at the remainder of the baryonic matter, it is primarily in the form
of helium, the next simplest element. The Sun’s atmosphere, for instance,
contains 70% hydrogen by mass, and 28% helium; only 2% is contributed by
more massive atoms. When astronomers look at a wide range of astronomical
objects – stars and interstellar gas clouds, for instance – they find a minimum
helium mass fraction of 24%. The baryonic component of the universe can
be described, to lowest order, as a mix of three parts hydrogen to one part
helium, with only minor contamination by heavier elements.
Another type of lepton, in addition to the electron, is the neutrino (ν).
The most poetic summary of the properties of the neutrino was made by
John Updike, in his poem “Cosmic Gall”6 :
Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.
In truth, Updike was using a bit of poetic license here. It is definitely true
that neutrinos have no charge.7 However, it is not true that neutrinos “do
not interact at all”; they actually are able to interact with other particles
via the weak nuclear force. The weak nuclear force, though, is very weak
indeed; a typical neutrino emitted by the Sun would have to pass through
a few parsecs of solid lead before having a 50% chance of interacting with a
lead atom. Since neutrinos pass through neutrino detectors with the same
facility with which they pass through the Earth, detecting neutrinos from
astronomical sources is difficult.
There are three types, or “flavors”, of neutrinos: electron neutrinos, muon
neutrinos, and tau neutrinos. What Updike didn’t know in 1960, when he
wrote his poem, is that some or all of the neutrino types probably have a small
6
c
From COLLECTED POEMS 1953-1993 by John Updike, °1993
by John Updike.
Used by permission of Alfred A. Knopf, a division of Random House, Inc.
7
Their name, given them by Enrico Fermi, means “little neutral one” in Italian.
25
2.4. TYPES OF PARTICLES
mass. The evidence for massive neutrinos comes indirectly, from the search
for neutrino oscillations. An “oscillation” is the transmutation of one flavor
of neutrino into another. The rate at which two neutrino flavors oscillate is
proportional to the difference of the squares of their masses. Observations of
neutrinos from the Sun are most easily explained if electron neutrinos (the
flavor emitted by the Sun) oscillate into some other flavor of neutrino, with
the difference in the squares of their masses being ∆(m2ν c4 ) ≈ 5 × 10−5 eV2 .
Observations of muon neutrinos created by cosmic rays striking the upper
atmosphere indicate that muon neutrinos oscillate into tau neutrinos, with
∆(m2ν c4 ) ≈ 3 × 10−3 eV2 for these two flavors. Unfortunately, knowing the
differences of the squares of the masses doesn’t tell us the values of the masses
themselves.
A particle which is known to be massless is the photon. Electromagnetic
radiation can be thought of either as a wave or as a stream of particles, called
photons. Light, when regarded as a wave, is characterized by its frequency
f or its wavelength λ = c/f . When light is regarded as a stream of photons,
each photon is characterized by its energy, Eγ = hf , where h = 2πh̄ is the
Planck constant. Photons of a wide range of energy, from radio to gamma
rays, pervade the universe. Unlike neutrinos, photons interact readily with
electrons, protons, and neutrons. For instance, photons can ionize an atom
by kicking an electron out of its orbit, a process known as photoionization.
Higher energy photons can break an atomic nucleus apart, a process known
as photodissociation.
Photons, in general, are easily created. One way to make photons is to
take a dense, opaque object – such as the filament of an incandescent lightbulb – and heat it up. If an object is opaque, then the protons, neutrons,
electrons, and photons which it contains frequently interact, and attain thermal equilibrium. When a system is in thermal equilibrium, the density of
photons in the system, as a function of photon energy, depends only on the
temperature T . It doesn’t matter whether the system is a tungsten filament,
or an ingot of steel, or a sphere of ionized hydrogen and helium. The energy density of photons in the frequency range f → f + df is given by the
blackbody function
ε(f )df =
8πh
f 3 df
,
c3 exp(hf /kT ) − 1
(2.25)
illustrated in Figure 2.7. The peak in the blackbody function occurs at
hfpeak ≈ 2.82kT . Integrated over all frequencies, equation (2.25) yields a
26
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
1
3 2
3
3
(c h /8πk T ) ε
1.5
.5
0
0
2
4
6 8
hf/kT
10 12
Figure 2.7: The energy distribution of a blackbody spectrum.
total energy density for blackbody radiation of
εγ = αT 4 ,
(2.26)
where
π2 k4
−16
α=
J m−3 K−4 .
(2.27)
3 3 = 7.56 × 10
15 h̄ c
The number density of photons in blackbody radiation can be computed from
equation (2.25) as
nγ = βT 3 ,
(2.28)
where
2.404 k 3
7
−3 −3
K .
(2.29)
3 3 = 2.03 × 10 m
2
π h̄ c
Division of equation (2.26) by equation (2.28) yields a mean photon energy of
Emean = hfmean ≈ 2.70kT , close to the peak in the spectrum. You have a temperature of 310 K, and you radiate an approximate blackbody spectrum, with
a mean photon energy of Emean ≈ 0.072 eV, corresponding to a wavelength of
λ ≈ 1.7 × 10−5 m, in the infrared. By contrast, the Sun produces an approximate blackbody spectrum with a temperature T¯ ≈ 5800 K. This implies
β=
2.4. TYPES OF PARTICLES
27
a mean photon energy Emean ≈ 1.3 eV, corresponding to λ ≈ 9.0 × 10−7 m,
in the near infrared. Note, however, that although the mean photon energy
in a blackbody spectrum is ∼ 3kT , Figure 2.7 shows us that there is a long
exponential tail to higher photon energies. A large fraction of the Sun’s output is at wavelengths of (4 → 7) × 10−7 m, which our eyes are equipped to
detect.
The most mysterious component of the universe is dark matter. When
observational astronomers refer to dark matter, they usually mean any massive component of the universe which is too dim to be detected readily using
current technology. Thus, stellar remnants such as white dwarfs, neutron
stars, and black holes are sometimes referred to as dark matter, since an
isolated stellar remnant is extremely faint and difficult to detect. Substellar
objects such as brown dwarfs are also referred to as dark matter, since brown
dwarfs, too low in mass for nuclear fusion to occur in their cores, are very
dim. Theoretical astronomers sometimes use a more stringent definition of
dark matter than observers do, defining dark matter as any massive component of the universe which doesn’t emit, absorb, or scatter light at all. 8 If
neutrinos have mass, for instance, as the recent neutrino oscillation results
indicate, they qualify as dark matter. In some extensions to the Standard
Model of particle physics, there exist massive particles which interact, like
neutrinos, only through the weak nuclear force and through gravity. These
particles, which have not yet been detected in the laboratory, are generically
referred to as Weakly Interacting Massive Particles, or WIMPs.
In this book, I will generally adopt the broader definition of dark matter
as something which is too dim for us to see, even with our best available technology. Detecting dark matter is, naturally, difficult. The standard method
of detecting dark matter is by measuring its gravitational effect on luminous
matter, just as the planet Neptune was first detected by its gravitational
effect on the planet Uranus. Although Neptune no longer qualifies as dark
matter, observations of the motions of stars within galaxies and of galaxies
within clusters indicate that there’s a significant amount of dark matter in
the universe. Exactly how much there is, and what it’s made of, is a topic of
great interest to cosmologists.
8
Using this definition, an alternate name for dark matter might be “transparent matter”
or “invisible matter”. However, the name “dark matter” has received the sanction of
history.
28
2.5
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
The universe is filled with a Cosmic Microwave Background
The discovery of the Cosmic Microwave Background (CMB) by Arno Penzias and Robert Wilson in 1965 has entered cosmological folklore. Using a
microwave antenna at Bell Labs, they found an isotropic background of microwave radiation. More recently, the Cosmic Background Explorer (COBE)
satellite has revealed that the Cosmic Microwave Background is exquisitely
well fitted by a blackbody spectrum (equation 2.25) with a temperature
T0 = 2.725 ± 0.001 K .
(2.30)
The energy density of the CMB is, from equation (2.26),
εγ = 4.17 × 10−14 J m−3 .
(2.31)
This is equivalent to roughly a quarter of an MeV per cubic meter of space.
The number density of CMB photons is, from equation (2.28),
nγ = 4.11 × 108 m−3 .
(2.32)
Thus, there are about 411 CMB photons in every cubic centimeter of the
universe at the present day. The mean energy of CMB photons, however, is
quite low, only
Emean = 6.34 × 10−4 eV .
(2.33)
This is too low in energy to photoionize an atom, much less photodissociate
a nucleus. About all they do, from a terrestrial point of view, is cause static
on television. The mean CMB photon energy corresponds to a wavelength
of 2 millimeters, in the microwave region of the electromagnetic spectrum –
hence the name “Cosmic Microwave Background”.
The existence of the CMB is a very important cosmological clue. In
particular, it is the clue which caused the Big Bang model for the universe
to be favored over the Steady State model. In a Steady State universe, the
existence of blackbody radiation at 2.725 K is not easily explained. In a Big
Bang universe, however, a cosmic background radiation arises naturally if the
universe was initially very hot as well as being very dense. If mass is conserved
in an expanding universe, then in the past, the universe was denser than it
is now. Assume that the early dense universe was very hot (T À 104 K, or
2.5. COSMIC MICROWAVE BACKGROUND
29
kT À 1eV ). At such high temperatures, the baryonic matter in the universe
was completely ionized, and the free electrons rendered the universe opaque.
A dense, hot, opaque body, as described in Section 2.4, produces blackbody
radiation. So, the early hot dense universe was full of photons, banging off
the electrons like balls in a pinball machine, with a spectrum typical of a
blackbody (equation 2.25). However, as the universe expanded, it cooled.
When the temperature dropped to ∼ 3000 K, ions and electrons combined
to form neutral atoms. When the universe no longer contained a significant
number of free electrons, the blackbody photons started streaming freely
through the universe, without further scattering off free electrons.
The blackbody radiation that fills the universe today can be explained
as a relic of the time when the universe was sufficiently hot and dense to
be opaque. However, at the time the universe became transparent, its temperature was ∼ 3000 K. The temperature of the CMB today is 2.725 K, a
factor of 1100 lower. The drop in temperature of the blackbody radiation is
a direct consequence of the expansion of the universe. Consider a region of
volume V which expands at the same rate as the universe, so that V ∝ a(t)3 .
The blackbody radiation in the volume can be thought as a photon gas with
energy density εγ = αT 4 . Moreover, since the photons in the volume have
momentum as well as energy, the photon gas has a pressure; the pressure of
a photon gas is Pγ = εγ /3. The photon gas within our imaginary box must
follow the laws of thermodynamics; in particular, the boxful of photons must
obey the first law
dQ = dE + P dV ,
(2.34)
where dQ is the amount of heat flowing into or out of the photon gas in the
volume V , dE is the change in the internal energy, P is the pressure, and
dV is the change in volume of the box. Since, in a homogeneous universe,
there is no net flow of heat (everything’s the same temperature, after all),
dQ = 0. Thus, the first law of thermodynamics, applied to an expanding
homogeneous universe, is
dE
dV
= −P (t)
.
dt
dt
(2.35)
Since, for the photons of the CMB, E = εγ V = αT 4 V and P = Pγ = αT 4 /3,
equation (2.35) can be rewritten in the form
Ã
α 4T
3 dT
dt
V +T
4 dV
dt
!
1
dV
= − αT 4
,
3
dt
(2.36)
30
CHAPTER 2. FUNDAMENTAL OBSERVATIONS
or
1 dV
1 dT
=−
.
(2.37)
T dt
3V dt
However, since V ∝ a(t)3 as the box expands, this means that the rate in
change of the photons’ temperature is related to the rate of expansion of the
universe by the relation
d
d
(ln T ) = − (ln a) .
dt
dt
(2.38)
This implies the simple relation T (t) ∝ a(t)−1 ; the temperature of the cosmic
background radiation has dropped by a factor of 1100 since the universe
became transparent because the scale factor a(t) has increased by a factor of
1100 since then. What we now see as a Cosmic Microwave Background was
once, at the time the universe became transparent, a Cosmic Near-Infrared
Background, with a temperature slightly cooler than the surface of the star
Betelgeuse.
The evidence cited so far can all be explained within the framework of
a Hot Big Bang model, in which the universe was originally very hot and
very dense, and since then has been expanding and cooling. The remainder
of this book will be devoted to working out the details of the Hot Big Bang
model which best fits the universe in which we live.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Bernstein (1995): The “Micropedia” which begins this text is a useful
overview of the contents of the universe and the forces which work
on them
Harrison (1987): The definitive treatment of Olbers’ paradox
Problems
(2.1) Suppose that in Sherwood Forest, the average radius of a tree is R =
1 m and the average number of trees per unit area is Σ = 0.005 m−2 .
If Robin Hood shoots an arrow in a random direction, how far, on
average, will it travel before it strikes a tree?
2.5. COSMIC MICROWAVE BACKGROUND
31
(2.2) Suppose you are in an infinitely large, infinitely old universe in which
the average density of stars is n? = 109 Mpc−3 and the average stellar
radius is equal to the Sun’s radius: R? = R¯ = 7 × 108 m. How far,
on average, could you see in any direction before your line of sight
struck a star? (Assume standard Euclidean geometry holds true in
this universe.) If the stars are clumped into galaxies with a density
ng = 1 Mpc−3 and average radius Rg = 2000 pc, how far, on average,
could you see in any direction before your line of sight hit a galaxy?
(2.3) Since you are made mostly of water, you are very efficient at absorbing
microwave photons. If you were in intergalactic space, approximately
how many CMB photons would you absorb per second? (If you like, you
may assume you are spherical.) What is the approximate rate, in watts,
at which you would absorb radiative energy from the CMB? Ignoring
other energy inputs and outputs, how long would it take the CMB to
raise your temperature by one nanoKelvin (10−9 K)? (You may assume
your heat capacity is the same as pure water, C = 4200 J kg −1 K−1 .)
(2.4) Suppose that the difference between the square of the mass of the
electron neutrino and that of the muon neutrino has the value [m(νµ )2 −
m(νe )2 ]c4 = 5 × 10−5 eV2 , and that the difference between the square
of the mass of the muon neutrino and that of the tau neutrino has the
value [m(ντ )2 − m(νµ )2 ]c4 = 3 × 10−3 eV2 . (This is consistent with the
observational results discussed in section 2.4.) What values of m(νe ),
m(νµ ), and m(ντ ) minimize the sum m(νe )+m(νµ )+m(ντ ), given these
constraints?
(2.5) A hypothesis once used to explain the Hubble relation is the “tired
light hypothesis”. The tired light hypothesis states that the universe
is not expanding, but that photons simply lose energy as they move
through space (by some unexplained means), with the energy loss per
unit distance being given by the law
dE
= −KE ,
dr
(2.39)
where K is a constant. Show that this hypothesis gives a distanceredshift relation which is linear in the limit z ¿ 1. What must the value
of K be in order to yield a Hubble constant of H0 = 70 km s−1 Mpc−1 ?
Chapter 3
Newton Versus Einstein
On cosmological scales (that is, on scales greater than 100 Mpc or so), the
dominant force determining the evolution of the universe is gravity. The weak
and strong nuclear forces are short-range forces; the weak force is effective
only on scales of `w ∼ 10−18 m or less, and the strong force on scales of `s ∼
10−15 m or less. Both gravity and electromagnetism are long range forces. On
small scales, gravity is negligibly small compared to electromagnetic forces;
for instance, the electrostatic repulsion between a pair of protons is larger by
a factor ∼ 1036 than the gravitational attraction between them. However, on
large scales, the universe is electrically neutral, so there are no electrostatic
forces on large scales. Moreover, intergalactic magnetic fields are sufficiently
small that magnetic forces are also negligibly tiny on cosmological scales.
Ironically then, gravity – the weakest of all forces from a particle physics
standpoint – is the force which determines the evolution of the universe on
large scales.
Note that in referring to gravity as a force, I am implicitly adopting
a Newtonian viewpoint. In physics, there are two useful ways of looking
at gravity – the Newtonian, or classical, viewpoint and the Einsteinian, or
general relativistic, viewpoint. In Isaac Newton’s view, as formulated by
his Laws of Motion and Law of Gravity, gravity is a force which causes
massive bodies to be accelerated. By contrast, in Einstein’s view, gravity
is a manifestation of the curvature of space-time. Although Newton’s view
and Einstein’s view are conceptually very different, in most cosmological
contexts they yield the same predictions. The Newtonian predictions differ
significantly from the predictions of general relativity only in the limit of deep
potential minima (to use Newtonian language) or strong spatial curvature (to
32
3.1. EQUIVALENCE PRINCIPLE
33
use general relativistic language). In these limits, general relativity yields the
correct result.
In the limit of shallow potential minima and weak spatial curvature, it
is permissible to switch back and forth between a Newtonian and a general
relativistic viewpoint, adopting whichever one is more convenient. I will
frequently adopt the Newtonian view of gravity in this book because, in
many contexts, it is mathematically simpler and conceptually more familiar.
The question of why it is possible to switch back and forth between the two
very different viewpoints of Newton and Einstein is an intriguing one, and
deserves closer investigation.
3.1
Equivalence principle
In Newton’s view of the universe, space is unchanging and Euclidean. In
Euclidean, or “flat”, space, all the axioms and theorems of plane geometry
(as codified by Euclid in the third century BC) hold true. In Euclidean space,
the shortest distance between two points is a straight line, the angles at the
vertices of a triangle sum to π radians, the circumference of a circle is 2π times
its radius, and so on, through all the other axioms and theorems you learned
in high school geometry. In Newton’s view, moreover, an object with no net
force acting on it moves in a straight line at constant speed. However, when
we look at objects in the Solar System such as planets, moons, comets, and
asteroids, we find that they move on curved lines, with constantly changing
speed. Why is this? Newton would tell us, “Their velocities are changing
because there is a force acting on them; the force called gravity.”
Newton devised a formula for computing the gravitational force between
two objects. Every object in the universe, said Newton, has a property which
we may call the “gravitational mass”. Let the gravitational masses of two
objects be Mg and mg , and let the distance between their centers be r. The
gravitational force acting between the two objects (assuming they are both
spherical) is
GMg mg
F =−
.
(3.1)
r2
The negative sign in the above equation indicates that gravity, in the Newtonian view, is always an attractive force, tending to draw two bodies closer
together.
34
CHAPTER 3. NEWTON VERSUS EINSTEIN
What is the acceleration which results from this gravitational force? Newton had something to say about that, as well. Every object in the universe,
said Newton, has a property which we may call the “inertial mass”. Let the
inertial mass of an object be mi . Newton’s second law of motion says that
force and acceleration are related by the equation
F = mi a .
(3.2)
In equations (3.1) and (3.2) I have distinguished, through the use of different
subscripts, between the gravitational mass mg and the inertial mass mi . One
of the fundamental principles of physics is that the gravitational mass and
the inertial mass of an object are identical:
mg = m i .
(3.3)
When you stop to think about it, this equality is a remarkable fact. The
property of an object that determines how strongly it is pulled on by the
force of gravity is equal to the property that determines its resistance to
acceleration by any force, not just the force of gravity. The equality of
gravitational mass and inertial mass is called the equivalence principle, and it
is the equivalence principle which led Einstein to devise his theory of general
relativity.
If the equivalence principle did not hold, then the gravitational acceleration of an object toward a mass Mg would be (combining equations 3.1 and
3.2)
¶
µ
GMg mg
,
(3.4)
a=− 2
r
mi
with the ratio mg /mi varying from object to object. However, when Galileo
dropped objects from towers and slid objects down inclined planes, he found
that the acceleration (barring the effects of air resistance and friction) was
always the same, regardless of the mass and composition of the object. The
magnitude of the gravitational acceleration close to the Earth’s surface is
2
g = GMEarth /rEarth
= 9.8 m s−2 . Modern tests of the equivalence principle,
which are basically more sensitive versions of Galileo’s experiments, reveal
that the inertial and gravitational masses are the same to within one part in
1012 .
To see how the equivalence principle led Einstein to devise his theory of
general relativity, let’s begin with a thought experiment of the sort Einstein
would devise. Suppose you wake up one morning to find that you have been
3.1. EQUIVALENCE PRINCIPLE
35
sealed up (bed and all) within an opaque, soundproof, hermetically sealed
box. “Oh no!” you say. “This is what I’ve always feared would happen.
I’ve been abducted by space aliens who are taking me away to their home
planet.” So startled are you by this realization, you drop your teddy bear.
Observing the falling bear, you find that it falls toward the floor of the box
with an acceleration a = 9.8 m s−2 . “Whew!” you say, with some relief. “At
least I am still on the Earth’s surface; they haven’t taken me away in their
spaceship yet.” At that moment, a window in the side of the box opens to
reveal (much to your horror) that you are inside an alien spaceship which is
being accelerated at 9.8 m s−2 by a rocket engine. When you drop a teddy
bear, or any other object, within a sealed box, the equivalence principle
permits two possible interpretations, with no way of distinguishing between
them. (1) The box is static, or moving with a constant velocity, and the
bear is being accelerated downward by a constant gravitational force. (2)
The bear is moving at a constant velocity, and the box is being accelerated
upward at a constant rate. The behavior of the bear in each case (Figure 3.1)
is identical. In each case, a big bear falls at the same rate as a little bear; in
each case, a bear stuffed with cotton falls at the same rate as a bear stuffed
with lead; and in each case, a sentient anglophone bear would say, “Oh,
bother. I’m weightless.” during the interval before it collides with the floor
of the box.1
Einstein’s insight, starting from the equivalence principle, led him to the
theory of general relativity. To understand Einstein’s thought processes,
imagine yourself back in the sealed box, being accelerated through interplanetary space at 9.8 m s−2 . You grab the flashlight that you keep on the bedside
table and shine a beam of light perpendicular to the acceleration vector (Figure 3.2). Since the box is accelerating upward, the path of the light beam
will appear to you to be bent downward, as the floor of the box rushes up
to meet the photons. However, thanks to the equivalence principle, we can
replace the accelerated box with a stationary box experiencing a constant
1
Note that the equivalence of the two boxes in Figure 3.1 depends on the gravitational
acceleration in the left-hand box being constant. In the real universe, though, gravitational accelerations are not exactly constant, but vary with position. For instance, the
gravitational acceleration near the Earth’s surface is a vector ~g (~r) which varies in direction
(always pointing toward the Earth’s center) and in magnitude (decreasing as the inverse
square of the distance from the Earth’s center). Thus, in the real universe, the equivalence
principle can only be applied to an infinitesimally small box – that is, a box so small that
the variation in ~g is too tiny to be measured.
36
CHAPTER 3. NEWTON VERSUS EINSTEIN
Figure 3.1: Equivalence principle (teddy bear version). The behavior of a
bear in an accelerated box (left) is identical to that of a bear being accelerated
by gravity (right).
3.1. EQUIVALENCE PRINCIPLE
37
Figure 3.2: Equivalence principle (photon version) The path followed by a
light beam in an accelerated box (left) is identical to the path followed by a
light beam subjected to gravitational acceleration (right).
38
CHAPTER 3. NEWTON VERSUS EINSTEIN
gravitational acceleration. Since there’s no way to distinguish between these
two cases, we are led to the conclusion that the paths of photons will be
curved downward in the presence of a gravitational field. Gravity affects
photons, Einstein concluded, even though they have no mass. Contemplating the curved path of the light beam, Einstein had one more insight. One of
the fundamental principles of optics is Fermat’s principle, which states that
light travels between two points along a path which minimizes the travel time
required.2 In a vacuum, where the speed of light is constant, this translates
into the requirement that light takes the shortest path between two points.
In Euclidean, or flat, space, the shortest path between two points is a straight
line. However, in the presence of gravity, the path taken by light is not a
straight line. Thus, Einstein concluded, space is not Euclidean.
The presence of mass, in Einstein’s view, causes space to be curved. In
fact, in the fully developed theory of general relativity, mass and energy
(which Newton thought of as two separate entities) are interchangeable, via
the famous equation E = mc2 . Moreover, space and time (which Newton
thought of as two separate entities) form a four-dimensional space-time. A
more accurate summary of Einstein’s viewpoint, then, is that the presence
of mass-energy causes space-time to be curved. We now have a third way of
thinking about the motion of the teddy bear in the box: (3) No forces are
acting on the bear; it is simply following a geodesic in curved space-time.3
We now have two ways of describing how gravity works.
The Way of Newton:
Mass tells gravity how to exert a force (F = −GM m/r 2 ),
Force tells mass how to accelerate (F = ma).
The Way of Einstein:
Mass-energy tells space-time how to curve,
Curved space-time tells mass-energy how to move.
4
Einstein’s description of gravity gives a natural explanation for the equivalence principle. In the Newtonian description of gravity, the equality of the
2
More generally, Fermat’s principle requires that the travel time be an extremum –
either a minimum or a maximum. In most situations, however, the path taken by light
minimizes the travel time rather than maximizing it.
3
In this context, the word “geodesic” is simply a shorter way of saying “the shortest
distance between two points”.
4
This pocket summary of general relativity was coined by the physicist John Wheeler,
who also popularized the term “black hole”.
3.2. DESCRIBING CURVATURE
39
Figure 3.3: A flat two-dimensional space.
gravitational mass and the inertial mass is a remarkable coincidence. However, in Einstein’s theory of general relativity, curvature is a property of
space-time itself. It then follows automatically that the gravitational acceleration of an object should be independent of mass and composition – it’s
just following a geodesic, which is dictated by the geometry of space-time.
3.2
Describing curvature
In developing a mathematical theory of general relativity, in which spacetime curvature is related to the mass-energy density, Einstein needed a way
of mathematically describing curvature. Since picturing the curvature of
a four-dimensional space-time is, to say the least, difficult, let’s start by
considering ways of describing the curvature of two-dimensional spaces, then
extend what we have learned to higher dimensions.
The simplest of two-dimensional spaces is a plane, on which Euclidean
geometry holds (Figure 3.3). On a plane, a geodesic is a straight line. If a
triangle is constructed on a plane by connecting three points with geodesics,
the angles at its vertices (α, β, and γ in Figure 3.3) obey the relation
α+β+γ =π ,
(3.5)
40
CHAPTER 3. NEWTON VERSUS EINSTEIN
where angles are measured in radians. On a plane, we can set up a cartesian
coordinate system, and assign to every point a coordinate (x, y). On a plane,
the Pythagorean theorem holds, so the distance ds between points (x, y) and
(x + dx, y + dy) is given by the relation5
ds2 = dx2 + dy 2 .
(3.6)
Stating that equation (3.6) holds true everywhere in a two-dimensional space
is equivalent to saying that the space is a plane. Of course, other coordinate
systems can be used, in place of cartesian coordinates. For instance, in a polar
coordinate system, the distance between points (r, θ) and (r + dr, θ + dθ) is
ds2 = dr2 + r2 dθ2 .
(3.7)
Although equations (3.6) and (3.7) are different in appearance, they both
represent the same flat geometry, as you can verify by making the simple
coordinate substitution x = r cos θ, y = r sin θ.
Now consider another simple two-dimensional space, the surface of a
sphere (Figure 3.4). On the surface of a sphere, a geodesic is a portion
of a great circle – that is, a circle whose center corresponds to the center
of the sphere. If a triangle is constructed on the surface of the sphere by
connecting three points with geodesics, the angles at its vertices (α, β, and
γ) obey the relation
α + β + γ = π + A/R2 ,
(3.8)
where A is the area of the triangle, and R is the radius of the sphere. All
spaces in which α + β + γ > π are called “positively curved” spaces. The
surface of a sphere is a positively curved two-dimensional space. Moreover,
it is a space where the curvature is homogeneous and isotropic; no matter
where you draw a triangle on the surface of a sphere, or how you orient it, it
must always satisfy equation (3.8).
On the surface of a sphere, we can set up a polar coordinate system by
picking a pair of antipodal points to be the “north pole” and “south pole”
and by picking a geodesic from the north to south pole to be the “prime
meridian”. If r is the distance from the north pole, and θ is the azimuthal
5
Starting with this equation, I am adopting the convention, commonly used among
relativists, that ds2 = (ds)2 , and not d(s2 ). Omitting the parentheses simply makes the
equations less cluttered.
3.2. DESCRIBING CURVATURE
41
Figure 3.4: A positively curved two-dimensional space.
angle measured relative to the prime meridian, then the distance ds between
a point (r, θ) and another nearby point (r +dr, θ +dθ) is given by the relation
ds2 = dr2 + R2 sin2 (r/R)dθ 2 .
(3.9)
Note that the surface of a sphere has a finite area, equal to 4πR 2 , and a
maximum possible distance between points; the distance between antipodal
points, at the maximum possible separation, is πR. By contrast, a plane
has infinite area, and has no upper limits on the possible distance between
points.6
In addition to flat spaces and positively curved spaces, there exist negatively curved spaces. An example of a negatively curved two-dimensional
space is the hyperboloid, or saddle-shape, shown in Figure 3.5. For illustrative purposes, I would like to show you a surface of constant negative curvature, just as the surface of a sphere has constant positive curvature.7 Un6
Since the publishers objected to producing a book of infinite size, Figure 3.3 actually
shows only a portion of a plane.
7
A space with constant curvature is one where the curvature is homogeneous and
isotropic.
42
CHAPTER 3. NEWTON VERSUS EINSTEIN
Figure 3.5: A negatively curved two-dimensional space.
fortunately, the mathematician David Hilbert proved that a two-dimensional
surface of constant negative curvature cannot be constructed in a threedimensional Euclidean space. The saddle-shape illustrated in Figure 3.5 has
constant curvature only in the central region, near the “seat” of the saddle.
Despite the difficulties in visualizing a surface of constant negative curvature, its properties can easily be written down. Consider a two-dimensional
surface of constant negative curvature, with radius of curvature R. If a triangle is constructed on this surface by connecting three points with geodesics,
the angles at its vertices (α,β, and γ) obey the relation
α + β + γ = π − A/R2 ,
(3.10)
where A is the area of the triangle.
On a surface of constant negative curvature, we can set up a polar coordinate system by choosing some point as the pole, and some geodesic leading
away from the pole as the prime meridian. If r is the distance from the pole,
and θ is the azimuthal angle measured relative to the prime meridian, then
the distance ds between a point (r, θ) and a nearby point (r + dr, θ + dθ) is
given by
ds2 = dr2 + R2 sinh2 (r/R)dθ 2 .
(3.11)
3.2. DESCRIBING CURVATURE
43
A surface of constant negative curvature has infinite area, and has no upper
limit on the possible distance between points.
Relations like those presented in equations (3.7), (3.9), and (3.11), which
give the distance ds between two nearby points in space, are known as metrics. In general, curvature is a local property. A tablecloth can be badly
rumpled at one end of the table and smooth at the other end; a bagel (or
other toroidal object) is negatively curved on part of its surface and positively curved on other portions.8 However, if you want a two-dimensional
space to be homogeneous and isotropic, there are only three possibilities that
fit the bill: the space can be uniformly flat, it can have uniform positive curvature, or it can have uniform negative curvature. Thus, if a two-dimensional
space has curvature which is homogeneous and isotropic, its geometry can
be specified by two quantities, κ, and R. The number κ, called the curvature
constant, is κ = 0 for a flat space, κ = +1 for a positively curved space,
and κ = −1 for a negatively curved space. If the space is curved, then the
quantity R, which has dimensions of length, is the radius of curvature.
The results for two-dimensional space can be extended straightforwardly
to three dimensions. A three-dimensional space, if its curvature is homogeneous and isotropic, must be flat, or have uniform positive curvature, or have
uniform negative curvature. If a three-dimensional space is flat (κ = 0), it
has the metric
ds2 = dx2 + dy 2 + dz 2 ,
(3.12)
expressed in cartesian coordinates, or
ds2 = dr2 + r2 [dθ2 + sin2 θdφ2 ] ,
(3.13)
expressed in spherical coordinates.
If a three-dimensional space has uniform positive curvature (κ = +1), its
metric is
ds2 = dr2 + R2 sin2 (r/R)[dθ 2 + sin2 θdφ2 ] .
(3.14)
A positively curved three-dimensional space has finite volume, just as a positively curved two-dimensional space has finite area. The point at r = πR is
the antipodal point to the origin, just as the south pole, at r = πR, is the
antipodal point to the north pole, at r = 0, on the surface of a sphere. By
traveling a distance C = 2πR, it is possible to “circumnavigate” a space of
uniform positive curvature.
8
You can test this assertion, if you like, by drawing triangles on a bagel.
44
CHAPTER 3. NEWTON VERSUS EINSTEIN
Finally, if a three-dimensional space has uniform negative curvature (κ =
−1), its metric is
ds2 = dr2 + R2 sinh2 (r/R)[dθ 2 + sin2 θdφ2 ] .
(3.15)
Like flat space, negatively curved space has infinite volume.
The three possible metrics for a homogeneous, isotropic, three-dimensional
space can be written more compactly in the form
ds2 = dr2 + Sκ (r)2 dΩ2 ,
(3.16)
dΩ2 ≡ dθ2 + sin2 θdφ2
(3.17)
where
and


 R sin(r/R)
(κ = +1)
(κ = 0)
Sκ (r) =  r

R sinh(r/R) (κ = −1) .
(3.18)
Note that in the limit r ¿ R, Sκ ≈ r, regardless of the value of κ. When
space is flat, or negatively curved, Sκ increases monotonically with r, with
Sκ → ∞ as r → ∞. By contrast, when space is positively curved, Sκ
increases to a maximum of Smax = R at r/R = π/2, then decreases again to
0 at r/R = π, the antipodal point to the origin.
The coordinate system (r, θ, φ) is not the only possible system. For instance, if we switch the radial coordinate from r to x ≡ Sκ (r), the metric
for a homogeneous, isotropic, three-dimensional space can be written in the
form
dx2
ds2 =
+ x2 dΩ2 .
(3.19)
1 − κx2 /R2
Although the metrics written in equations (3.16) and (3.19) appear different
on the page, they represent the same homogeneous, isotropic spaces. They
merely have a different functional form because of the different choice of
radial coordinates.
3.3
The Robertson-Walker metric
So far, we’ve only considered the metrics for simple two-dimensional and
three-dimensional spaces. However, relativity teaches us that space and time
3.3. ROBERTSON-WALKER METRIC
45
together comprise a four-dimensional space-time. Just as we can compute
the distance between two points in space using the appropriate metric for
that space, so we can compute the four-dimensional distance between two
events in space-time. Consider two events, one occurring at the space-time
location (t, r, θ, φ), and another occurring at the space-time location (t +
dt, r + dr, θ + dθ, φ + dφ). According to the laws of special relativity, the
space-time separation between these two events is
ds2 = −c2 dt2 + dr2 + r2 dΩ2 .
(3.20)
The metric given in equation (3.20) is called the Minkowski metric, and the
space-time which it describes is called Minkowski space-time. Note, from a
comparison with equation (3.16), that the spatial component of Minkowski
space-time is Euclidean, or flat.
A photon’s path through space-time is a four-dimensional geodesic – and
not just any geodesic, mind you, but a special variety called a null geodesic.
A null geodesic is one for which, along every infinitesimal segment of the
photon’s path, ds = 0. In Minkowski space-time, then, a photon’s trajectory
obeys the relation
ds2 = 0 = −c2 dt2 + dr2 + r2 dΩ2 .
(3.21)
If the photon is moving along a radial path, towards or away from the origin,
this means, since θ and φ are constant,
or
c2 dt2 = dr2 ,
(3.22)
dr
= ±c .
dt
(3.23)
The Minkowski metric of equation (3.20) applies only within the context
of special relativity, so called because it deals with the special case in which
space-time is not curved by the presence of mass and energy. Without any
gravitational effects, Minkowski space-time is flat and static. When gravity
is added, however, the permissible space-times are more interesting. In the
1930’s, the physicists Howard Robertson and Arthur Walker asked “What
form can the metric of space-time assume if the universe is spatially homogeneous and isotropic at all time, and if distances are allowed to expand
46
CHAPTER 3. NEWTON VERSUS EINSTEIN
(or contract) as a function of time?” The metric which they derived, independently of each other, is called the Robertson-Walker metric. It is most
generally written in the form
2
2
2
ds = −c dt + a(t)
2
"
dx2
+ x2 dΩ2
2
2
1 − κx /R0
#
.
(3.24)
Note that the spatial component of the Robertson-Walker metric consists
of the spatial metric for a uniformly curved space of radius R0 (compare
equation 3.19), scaled by the square of the scale factor a(t). The scale factor, first introduced in Section 2.3, describes how distances in a homogeneous,
isotropic universe expand or contract with time. The Robertson-Walker metric can also be written in the form (see equation 3.16)
h
i
ds2 = −c2 dt2 + a(t)2 dr2 + Sκ (r)2 dΩ2 ,
(3.25)
with the function Sκ (r) for the three different types of curvature given by
equation (3.18).
The time variable t in the Robertson-Walker metric is the cosmological
proper time, called the cosmic time for short, and is the time measured by
an observer who sees the universe expanding uniformly around him. The
spatial variables (x, θ, φ) or (r, θ, φ) are called the comoving coordinates of a
point in space; if the expansion of the universe is perfectly homogeneous and
isotropic, the comoving coordinates of any point remain constant with time.
The assumption of homogeneity and isotropy is a very powerful one. If
the universe is perfectly homogeneous and isotropic, then everything we need
to know about its geometry is contained within a(t), κ, and R0 . The scale
factor a(t) is a dimensionless function of time which describes how distances
grow or decrease with time; it is normalized so that a(t0 ) = 1 at the present
moment. The curvature constant κ is a dimensionless number which can take
on one of three discrete values: κ = 0 if the universe is spatially flat, κ = −1
if the universe has negative spatial curvature, and κ = +1 if the universe
has positive spatial curvature. The radius of curvature R0 has dimensions
of length, and gives the radius of curvature of the universe at the present
moment. Much of modern cosmology, as we’ll see in later chapters, is devoted
in one way or another to finding the values of a(t), κ, and R0 . The assumption
of spatial homogeneity and isotropy is so powerful, Robertson and Walker
made it in the 1930’s, long before the available observational evidence gave
any support for such an assumption. If homogeneity and isotropy did not
3.4. PROPER DISTANCE
47
exist, as Voltaire might have said, it would be necessary to invent them –
at least if your desire is to have a simple, analytically tractable form for the
metric of space-time.
In truth, the observations reveal that the universe is not homogeneous
and isotropic on small scales. Thus, the Robertson-Walker metric is only
an approximation which holds good on large scales. On smaller scales, the
universe is “lumpy”, and hence does not expand uniformly. Small, dense
lumps, such as humans, teddy bears, and interstellar dust grains, are held
together by electromagnetic forces, and hence do not expand. Larger lumps,
as long as they are sufficiently dense, are held together by their own gravity,
and hence do not expand. Examples of such gravitationally bound systems
are planetary systems (such as the Solar System in which we live), galaxies
(such as the Galaxy in which we live), and clusters of galaxies (such as the
Local Group in which we live). It’s only on scales larger than ∼ 100 Mpc
that the expansion of the universe can be treated as the ideal, homogeneous,
isotropic expansion described by the single scale factor a(t).
3.4
Proper distance
Consider a galaxy which is far away from us – sufficiently far away that
we may ignore the small scale perturbations of space-time and adopt the
Robertson-Walker metric. One question we may ask is, “Exactly how far
away is this galaxy?” In an expanding universe, the distance between two
objects is increasing with time. Thus, if we want to assign a spatial distance
D between two objects, we must specify the time t at which the distance is
the correct one. Suppose that you are at the origin, and that the galaxy which
you are observing is at a comoving coordinate position (r, θ, φ), as illustrated
in Figure 3.6. The proper distance dp (t) between two points is equal to the
length of the spatial geodesic between them when the scale factor is fixed
at the value a(t). The proper distance between the observer and galaxy in
Figure 3.6 can be found using the Robertson-Walker metric at a fixed time
t:
ds2 = a(t)2 [dr2 + Sκ (r)2 dΩ2 ] .
(3.26)
Along the spatial geodesic between the observer and galaxy, the angle (θ, φ)
is constant, and thus
ds = a(t)dr .
(3.27)
48
CHAPTER 3. NEWTON VERSUS EINSTEIN
Figure 3.6: An observer at the origin observes a galaxy at coordinate position
(r, θ, φ). A photon emitted by the galaxy at cosmic time te reaches the
observer at cosmic time t0 .
49
3.4. PROPER DISTANCE
The proper distance dp is found by integrating over the radial comoving
coordinate r:
Z r
dp (t) = a(t)
dr = a(t)r .
(3.28)
0
Alternatively, if you wish to use the spatial coordinates (x, θ, φ) instead of
(r, θ, φ), where x = Sκ (r), you may invert the relations of equation (3.18) to
find

−1

 a(t)R0 sin (x/R0 )
(κ = +1)
(κ = 0)
dp (t) = a(t)r(x) =  a(t)x

a(t)R0 sinh−1 (x/R0 ) (κ = −1) .
(3.29)
Because the proper distance has the form dp (t) = a(t)r, with the comoving
coordinate r constant with time, the rate of change for the proper distance
between us and a distant galaxy is
ȧ
d˙p = ȧr = dp .
a
(3.30)
Thus, at the current time (t = t0 ), there is a linear relation between the
proper distance to a galaxy and its recession speed:
where
vp (t0 ) = H0 dp (t0 ) ,
(3.31)
vp (t0 ) ≡ d˙p (t0 )
(3.32)
and
ȧ
.
(3.33)
a t=t0
In a sense, this is just a repetition of what was demonstrated in Section 2.3;
if the distance between points is proportional to a(t), there will be a linear
relation between the relative velocity of two points and the distance between
them. Now, however, we are interpreting the change in distance between
widely separated galaxies as being associated with the expansion of space.
As the distance between galaxies increases, the radius of curvature of the
universe, R(t) = a(t)R0 , increases at the same rate.
Some cosmology books will contain a statement like “As space expands,
it drags galaxies away from each other.” Statements of this sort are misleading, since they make galaxies appear to be entirely passive. On the other
hand, a statement like “As galaxies move apart, they drag space along with
H0 =
µ ¶
50
CHAPTER 3. NEWTON VERSUS EINSTEIN
them” would be equally misleading, since it makes space appear to be entirely passive. As the theory of general relativity points out, space-time and
mass-energy are intimately linked. Yes, the curvature of space-time does tell
mass-energy how to move, but then it’s mass-energy which tells space-time
how to curve.
The linear velocity-distance relation given in equation (3.31) implies that
points separated by a proper distance greater than a critical value
dH (t0 ) ≡ c/H0 ,
(3.34)
generally called the Hubble distance, will have
vp = d˙p > c .
(3.35)
Using the observationally determined value of H0 = 70 ± 7 km s−1 Mpc−1 , the
current value of the Hubble distance in our universe is
dH (t0 ) = 4300 ± 400 Mpc .
(3.36)
Thus, galaxies farther than 4300 megaparsecs from us are currently moving
away from us at speeds greater than that of light. Cosmological innocents
sometimes exclaim, “Gosh! Doesn’t this violate the law that massive objects
can’t travel faster than the speed of light?” Actually, it doesn’t. The speed
limit that states that massive objects must travel with v < c relative to
each other is one of the results of special relativity, and refers to the relative
motion of objects within a static space. In the context of general relativity,
there is no objection to having two points moving away from each other at
superluminal speed due to the expansion of space.
When we observe a distant galaxy, we know its angular position very
well, but not its distance. That is, we can point in its direction, but we don’t
know its current proper distance dp (t0 ) – or, for that matter, its comoving
coordinate distance r. We can, however, measure the redshift z of the light
we receive from the galaxy. Although the redshift doesn’t tell us the proper
distance to the galaxy, it does tell us what the scale factor a was at the
time the light from that galaxy was emitted. To see the link between a and
z, consider the galaxy illustrated in Figure 3.6. Light that was emitted by
the galaxy at a time te is observed by us at a time t0 . During its travel
from the distant galaxy to us, the light traveled along a null geodesic, with
51
3.4. PROPER DISTANCE
ds = 0. The null geodesic has θ and φ constant.9 Thus, along the light’s null
geodesic,
c2 dt2 = a(t)2 dr2 .
(3.37)
Rearranging this relation, we find
c
dt
= dr .
a(t)
(3.38)
In equation (3.38), the left-hand side is a function only of t, and the righthand side is independent of t. Suppose the distant galaxy emits light with
a wavelength λe , as measured by an observer in the emitting galaxy. Fix
your attention on a single wave crest of the emitted light. The wave crest is
emitted at a time te and observed at a time t0 , such that
c
Z
t0
te
Z r
dt
=
dr = r .
a(t)
0
(3.39)
The next wave crest of light is emitted at a time te + λe /c, and is observed
at a time t0 + λ0 /c, where, in general, λ0 6= λe . For the second wave crest,
c
Z
t0 +λ0 /c
te +λe /c
Z r
dt
=
dr = r .
a(t)
0
(3.40)
Comparing equations (3.39) and (3.40), we find that
Z
t0
te
Z t0 +λ0 /c
dt
dt
=
.
a(t)
te +λe /c a(t)
(3.41)
That is, the integral of dt/a(t) between the time of emission and the time
of observation is the same for every wave crest in the emitted light. If we
subtract the integral
Z t0
dt
(3.42)
te +λe /c a(t)
from each side of equation (3.41), we find the relation
Z
9
te +λe /c
te
Z t0 +λ0 /c
dt
dt
=
.
a(t)
a(t)
t0
(3.43)
In a homogeneous, isotropic universe there’s no reason for the light to swerve to one
side or the other.
52
CHAPTER 3. NEWTON VERSUS EINSTEIN
That is, the integral of dt/a(t) between the emission of successive wave crests
is equal to the integral of dt/a(t) between the observation of successive wave
crests. This relation becomes still simpler when we realize that during the
time between the emission or observation of two wave crests, the universe
doesn’t have time to expand by a significant amount. The time scale for
expansion of the universe is the Hubble time, H0−1 ≈ 14 Gyr. The time
between wave crests, for visible light, is λ/c ≈ 2 × 10−15 s ≈ 10−32 H0−1 .
Thus, a(t) is effectively constant in the integrals of equation (3.43). Thus,
we may write
1 Z te +λe /c
1 Z t0 +λ0 /c
dt =
dt ,
(3.44)
a(te ) te
a(t0 ) t0
or
λ0
λe
=
.
(3.45)
a(te )
a(t0 )
Using the definition of redshift, z = (λ0 − λe )/λe , we find that the redshift
of light from a distant object is related to the expansion factor at the time
it was emitted via the equation
1+z =
1
a(t0 )
=
.
a(te )
a(te )
(3.46)
Here, I have used the usual convention that a(t0 ) = 1.
Thus, if we observe a galaxy with a redshift z = 2, we are observing it
as it was when the universe had a scale factor a(te ) = 1/3. The redshift
we observe for a distant object depends only on the relative scale factors
at the time of emission and the time of observation. It doesn’t depend on
how the transition between a(te ) and a(t0 ) was made. It doesn’t matter if
the expansion was gradual or abrupt; it doesn’t matter if the transition was
monotonic or oscillatory. All that matters is the scale factors at the time of
emission and the time of observation.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Harrison (2000), ch. 10 – 12: Curved space and relativity (both special
and general)
53
3.4. PROPER DISTANCE
Narlikar (2002), ch. 2 – 3: Delves deeper into general relativity while discussing the Robertson-Walker metric
Peacock (2000), ch. 3.1: Derivation and discussion of the Robertson-Walker
metric
Rich (2001), ch. 3: Coordinates and metrics in the context of general relativity
Problems
(3.1) What evidence can you provide to support the assertion that the universe is electrically neutral on large scales?
(3.2) Suppose you are a two-dimensional being, living on the surface of a
sphere with radius R. An object of width ds ¿ R is at a distance
r from you (remember, all distances are measured on the surface of
the sphere). What angular width dθ will you measure for the object?
Explain the behavior of dθ as r → πR.
(3.3) Suppose you are still a two-dimensional being, living on the same
sphere of radius R. Show that if you draw a circle of radius r, the
circle’s circumference will be
C = 2πR sin(r/R) .
(3.47)
Idealize the Earth as a perfect sphere of radius R = 6371 km. If you
could measure distances with an error of ±1 meter, how large a circle
would you have to draw on the Earth’s surface to convince yourself
that the Earth is spherical rather than flat?
(3.4) Consider an equilateral triangle, with sides of length L, drawn on
a two-dimensional surface of constant curvature. Can you draw an
equilateral triangle of arbitrarily large area A on a surface with κ = +1
and radius of curvature R? If not, what is the maximum possible value
of A? Can you draw an equilateral triangle of arbitrarily large area A
on a surface with κ = 0? If not, what is the maximum possible value
of A? Can you draw an equilateral triangle of arbitrarily large area A
on a surface with κ = −1 and radius of curvature R? If not, what is
the maximum possible value of A?
54
CHAPTER 3. NEWTON VERSUS EINSTEIN
(3.5) By making the substitutions x = r sin θ cos φ, y = r sin θ sin φ, and
z = r cos θ, demonstrate that equations (3.12) and (3.13) represent the
same metric.
Chapter 4
Cosmic Dynamics
In a universe which is homogeneous and isotropic, but which is allowed to
expand or contract with time, everything you need to know about the curvature is given by κ, R0 , and a(t). The curvature constant κ gives the sign
of the curvature: positive (κ = +1), negative (κ = −1), or flat (κ = 0). If κ
is non-zero, then R0 is the radius of curvature of the universe, as measured
at the present moment (t = t0 ). Finally, the scale factor a(t) tells how distances in the universe increase with time as the universe expands, or decrease
with time as the universe contracts. The scale factor is normalized so that
a(t0 ) = 1 at the present moment.
The idea that the universe could be curved, or non-Euclidean, actually
long predates Einstein’s theory of general relativity. As early as 1829, half
a century before Einstein’s birth, Nikolai Ivanovich Lobachevski, one of the
founders of non-Euclidean geometry, proposed observational tests to demonstrate whether the universe was curved. In principle, measuring the curvature
of the universe is simple; in practice it is much more difficult. In principle,
we could determine the curvature by drawing a really, really big triangle, and
measuring the angles α, β, and γ at the vertices. Equations (3.5), (3.8), and
(3.10) generalize to the equation
α+β+γ =π+
κA
,
R02
(4.1)
where A is the area of the triangle. Therefore, if the α + β + γ > π radians,
the universe is positively curved, and if α + β + γ < π radians, the universe
is negatively curved. If, in addition, you measure the area of the triangle,
you can determine the radius of curvature R0 . Unfortunately for this elegant
55
56
CHAPTER 4. COSMIC DYNAMICS
Figure 4.1: A two-dimensional positively curved universe, demonstrating how
an observer in such a universe could see multiple images of the same galaxy.
geometric plan, the area of the biggest triangle we can draw is much smaller
than R02 , and the deviation of α + β + γ from π radians would be too small
to measure.
About all we can conclude from geometric arguments is that if the universe is positively curved, it can’t have a radius of curvature R0 that is
significantly smaller than the current Hubble distance, c/H0 ≈ 4300 Mpc.
To understand why this is so, recall that if our universe is positively curved,
it has finite size, with a circumference currently equal to C0 = 2πR0 . In
the past, since our universe is expanding, its circumference was even smaller.
Thus, if the current circumference C0 is less than ct0 , then photons will have
had time to circumnavigate the universe. If C0 ¿ ct0 ∼ c/H0 , then photons
will have had time to circumnavigate the universe many times. To take an
extreme example, suppose the universe were positively curved with a circumference of only 10 million light years (roughly 3 Mpc). The two-dimensional
analog to such a universe is shown in Figure 4.1. Looking toward the galaxy
4.1. FRIEDMANN EQUATION
57
M31, which is 2 million light years away from us, we would see one image of
M31, comprised of photons which had traveled 2 million light years, showing
M31 as it was 2 million years ago. We would also see another image, comprised of photons which had traveled 12 million light years, showing M31 as
it was 12 million years ago. And so on. Moreover, looking in the exact opposite direction to M31, we would see an image of M31, comprised of photons
which had traveled 8 million light years, showing M31 as it was 8 million
years ago. We would also see another image, comprised of photons which
had traveled 18 million light years, showing M31 as it was 18 million years
ago.1 And so on. Since we don’t see periodicities of this sort, we conclude
that if the universe is positively curved, its radius of curvature R0 must be
very large – comparable to or larger than the current Hubble distance c/H0 .
4.1
The Friedmann equation
Although 19th century mathematicians and physicists, such as Lobachevski,
were able to conceive of curved space, it wasn’t until Albert Einstein first
published his theory of general relativity in 1915 that the curvature of spacetime was linked to its mass-energy content. The key equation of general
relativity is Einstein’s field equation, which is the relativistic equivalent of
Poisson’s equation in Newtonian dynamics. Poisson’s equation,
∇2 Φ = 4πGρ ,
(4.2)
gives a mathematical relation between the gravitational potential Φ at a
point in space and the mass density ρ at that point. By taking the gradient
of the potential, you determine the acceleration, and then can compute the
trajectory of objects moving freely through space. Einstein’s field equation,
by contrast, gives a mathematical relation between the metric of space-time
at a point and the energy and pressure at that space-time point. The trajectories of of freely moving objects then correspond to geodesics in curved
space-time.
In a cosmological context, Einstein’s field equations can be used to find
the linkage between a(t), κ, and R0 , which describe the curvature of the universe, and the energy density ε(t) and pressure P (t) of the contents of the
1
This assumes that 2πR0 /c ¿ H0−1 , and that the universe therefore doesn’t expand
significantly as a photon goes once or twice around the universe.
58
CHAPTER 4. COSMIC DYNAMICS
universe. The equation which links together a(t), κ, R0 , and ε(t) is known as
the Friedmann equation, after Alexander Alexandrovich Friedmann, who first
derived the equation in 1922. Friedmann actually started his scientific career
as a meteorologist. Later, however, he taught himself general relativity, and
used Einstein’s field equations to describe how a spatially homogeneous and
isotropic universe expands or contracts as a function of time. It is intriguing
to note that Friedmann published his first results, implying an expanding or
contracting universe, seven years before Hubble published Hubble’s Law in
1929. Unfortunately, Friedmann’s papers received little notice at first. Even
Einstein initially dismissed Friedmann’s work as a mathematical curiosity,
unrelated to the universe we actually live in. It wasn’t until Hubble’s results
were published that Einstein acknowledged the reality of the expanding universe. Alas, Friedmann did not live to see his vindication; he died of typhoid
fever in 1925, when he was only 37 years old.
Friedmann derived his eponymous equation starting from Einstein’s field
equation, using the full power of general relativity. Even without bringing
relativity into play, some (though not all) of the aspects of the Friedmann
equation can be understood with the use of purely Newtonian dynamics. To
see how the expansion or contraction of the universe can be viewed from a
Newtonian viewpoint, I will first derive the non-relativistic equivalent of the
Friedmann equation, starting from Newton’s Law of Gravity and Second Law
of Motion. Then I will state (without proof) the modifications that must be
made to find the more correct, general relativistic form of the Friedmann
equation.
To begin, consider a homogeneous sphere of matter, with total mass Ms
constant with time (Figure 4.2). The sphere is expanding or contracting
isotropically, so that its radius Rs (t) is increasing or decreasing with time.
Place a test mass, of infinitesimal mass m, at the surface of the sphere. The
gravitational force F experienced by the test mass will be, from Newton’s
Law of Gravity,
GMs m
.
(4.3)
F =−
Rs (t)2
The gravitational acceleration at the surface of the sphere will then be, from
Newton’s Second Law of Motion,
d 2 Rs
GMs
=−
.
2
dt
Rs (t)2
(4.4)
59
4.1. FRIEDMANN EQUATION
r(t)
Figure 4.2: A sphere of radius Rs (t) and mass Ms , expanding or contracting
under its own gravity.
Multiply each side of the equation by dRs /dt and integrate to find
1
2
Ã
dRs
dt
!2
=
GMs
+U ,
Rs (t)
(4.5)
where U is a constant of integration. Equation (4.5) simply states that the
sum of the kinetic energy per unit mass,
Ekin
1
=
2
Ã
dRs
dt
!2
,
(4.6)
and the gravitational potential energy per unit mass,
Epot = −
GMs
,
Rs (t)
(4.7)
is constant for a bit of matter at the surface of a sphere, as the sphere expands
or contracts under its own gravitational influence.
Since the mass of the sphere is constant as it expands or contracts, we
may write
4π
ρ(t)Rs (t)3 .
(4.8)
Ms =
3
Since the expansion is isotropic about the sphere’s center, we may write the
radius Rs (t) in the form
Rs (t) = a(t)rs ,
(4.9)
60
CHAPTER 4. COSMIC DYNAMICS
where a(t) is the scale factor and rs is the comoving radius of the sphere.
In terms of ρ(t) and a(t), the energy conservation equation (4.5) can be
rewritten in the form
1 2 2 4π 2
r ȧ =
Grs ρ(t)a(t)2 + U .
2 s
3
(4.10)
Dividing each side of equation (4.10) by rs2 a2 /2 yields the equation
µ ¶2
ȧ
a
=
8πG
2U 1
ρ(t) + 2
.
3
rs a(t)2
(4.11)
Equation (4.11) gives the Friedmann equation in its Newtonian form.
Note that the time derivative of the scale factor only enters into equation (4.11) as ȧ2 ; a contracting sphere (ȧ < 0) is simply the time reversal
of an expanding sphere (ȧ > 0). Let’s concentrate on the case of a sphere
which is expanding, analogous to the expanding universe in which we find
ourselves. The future of the expanding sphere falls into one of three classes,
depending on the sign of U . First, consider the case U > 0. In this case, the
right hand side of equation (4.11) is always positive. Therefore, ȧ2 is always
positive, and the expansion of the sphere never stops. Second, consider the
case U < 0. In this case, the right hand side of equation (4.11) starts out
positive. However, at a maximum scale factor
amax = −
GMs
,
U rs
(4.12)
the right hand side will equal zero, and expansion will stop. Since ä will still
be negative, the sphere will then contract. Third, and finally, consider the
case U = 0. This is the boundary case in which ȧ → 0 as t → ∞ and ρ → 0.
The three possible fates of an expanding sphere in a Newtonian universe
are analogous to the three possible fates of a ball thrown upward from the
surface of the Earth. First, the ball can be thrown upward with a speed
greater than the escape speed; in this case, the ball continues to go upward
forever. Second, the ball can be thrown upward with a speed less than
the escape speed; in this case, the ball reaches a maximum altitude, then
falls back down. Third, and finally, the ball can be thrown upward with a
speed exactly equal to the escape speed; in this case, the speed of the ball
approaches zero as t → ∞.
The Friedmann equation in its Newtonian form (equation 4.11) is useful
in picturing how isotropically expanding objects behave under the influence
4.1. FRIEDMANN EQUATION
61
of their self-gravity. However, its application to the real universe must be regarded with considerable skepticism. First of all, a spherical volume of finite
radius Rs cannot represent a homogeneous, isotropic universe. In a finite
spherical volume, there exists a special location (the center of the sphere),
violating the principle of homogeneity, and at any point there exists a special
direction (the direction pointing toward the center), violating the principle
of isotropy. We may instead regard the sphere of radius Rs as being carved
out of an infinite, homogeneous, isotropic universe. In that case, Newtonian
dynamics tell us that the gravitational acceleration inside a hollow spherically symmetric shell is equal to zero. We divide up the region outside the
sphere into concentric shells, and thus conclude that the test mass m at Rs
experiences no net acceleration from matter at R > Rs . Unfortunately, a
Newtonian argument of this sort assumes that space is Euclidean. A derivation of the correct Friedmann equation, including the possibility of spatial
curvature, has to begin with Einstein’s field equations.
The correct form of the Friedmann equation, including all general relativistic effects, is
µ ¶2
ȧ
κc2 1
8πG
ε(t)
−
.
(4.13)
=
a
3c2
R02 a(t)2
Note the changes made in going from the Newtonian form of the Friedmann
equation (equation 4.11) to the correct relativistic form (equation 4.13). The
first change is that the mass density ρ has been replaced by an energy density
ε divided by the square of the speed of light. One of Einstein’s insights was
that in determining the gravitational influence of a particle, the important
quantity was not its mass m but its energy,
E = (m2 c4 + p2 c2 )1/2 .
(4.14)
Here p is the momentum of the particle as seen by an observer at the particle’s location who sees the universe expanding isotropically around her. Any
motion which a particle has, in addition to the motion associated with the
expansion or contraction of the universe, is called the particle’s peculiar motion.2 If a massive particle is non-relativistic – that is, if its peculiar velocity
v is much less than c – then its peculiar momentum will be p ≈ mv, and its
2
The adjective “peculiar” comes from the Latin “peculium”, meaning “private property”. The peculiar motion of a particle is thus the motion which belongs to the particle
alone, and not to the global expansion or contraction of the universe.
62
CHAPTER 4. COSMIC DYNAMICS
energy will be
1
Enon−rel ≈ mc2 (1 + v 2 /c2 ) ≈ mc2 + mv 2 .
2
(4.15)
Thus, if the universe contained only massive, slowly moving particles, then
the energy density ε would be nearly equal to ρc2 , with only a small correction
for the kinetic energy mv 2 /2 of the particles. However, photons and other
massless particles also have an energy,
Erel = pc = hf ,
(4.16)
which also contributes to the energy density ε. Not only do photons respond
to the curvature of space-time, they also contribute to it.
The second change that must be made in going from the Newtonian form
of the Friedmann equation to the correct relativistic form is making the
substitution
2U
κc2
=− 2 .
(4.17)
rs2
R0
In the context of general relativity, the curvature κ is related to the Newtonian energy U of a test mass. The case with U < 0 corresponds to positive
curvature (κ = +1), while the case with U > 0 corresponds to negative curvature (κ = −1). The special case with U = 0 corresponds to the special
case where the space is perfectly flat (κ = 0). Although I have not given the
derivation of the Friedmann equation in the general relativistic case, it makes
sense that the curvature, given by κ and R0 , the expansion rate, given by a(t),
and the energy density ε should be bound up together in the same equation.
After all, in Einstein’s view, the energy density of the universe determines
both the curvature of space and the overall dynamics of the expansion.
The Friedmann equation is a Very Important Equation in cosmology.3
However, if we want to apply the Friedmann equation to the real universe,
we must have some way of tying it to observable properties. For instance,
the Friedmann equation can be tied to the Hubble constant, H0 . Remember,
in a universe whose expansion (or contraction) is described by a scale factor
a(t), there’s a linear relation between recession speed v and proper distance
d:
v(t) = H(t)d(t) ,
(4.18)
3
You should consider writing it in reverse on your forehead so that you can see it every
morning in the mirror when you comb your hair.
63
4.1. FRIEDMANN EQUATION
where H(t) ≡ ȧ/a. Thus, the Friedmann equation can be rewritten in the
form
κc2
8πG
ε(t)
−
.
(4.19)
H(t)2 =
3c2
R02 a(t)2
At the present moment,
H0 = H(t0 ) =
ȧ
a
µ ¶
t=t0
= 70 ± 7 km s−1 Mpc−1 .
(4.20)
As an etymological aside, I should point out that the time-varying function
H(t) is generally known as the “Hubble parameter”, while H0 , the value of
H(t) at the present day, is known as the “Hubble constant”.
The Friedmann equation evaluated at the present moment is
H02 =
8πG
κc2
ε
−
,
0
3c2
R02
(4.21)
using the convention that a subscript “0” indicates the value of a timevarying quantity evaluated at the present. Thus, the Friedmann equation
gives a relation among H0 , which tells us the current rate of expansion,
ε0 , which tells us the current energy density, and κ/R02 , which tells us the
current curvature. Due to the difficulty of measuring the curvature directly
by geometric means, it is useful to have an indirect method of determining κ
and R0 . If we were able to measure H0 and ε0 with high precision, we could
use equation (4.21) to determine the curvature. Even without knowledge of
the current density ε0 , we can use equation (4.21) to place a lower limit on
R0 in a negatively curved universe. If we assume ε0 is non-negative, then for
a given value of H0 , the product κ/R02 is minimized in the limit ε0 → 0. In
the limit of a totally empty universe, with no energy content, the curvature
is negative, with a radius of curvature
R0 (min) = c/H0 .
(4.22)
This is the minimum radius of curvature which a negatively curved universe
can have, assuming that general relativity correctly describes the curvature.
Since we know that the universe contains matter and radiation, and hence
that ε0 > 0, the radius of curvature must be greater than the Hubble distance
if the universe is negatively curved.4
4
We also know from observations, as discussed earlier, that the radius of curvature
must be comparable to or greater than the Hubble distance if the universe is positively
curved.
64
CHAPTER 4. COSMIC DYNAMICS
As we have seen, the Friedmann equation can generally be written as
κc2
8πG
ε(t) − 2
,
H(t) =
3c2
R0 a(t)
2
(4.23)
for all universes with a Robertson-Walker metric whose expansion or contraction is governed by the rules of general relativity. In a spatially flat universe
(κ = 0), the Friedmann equation takes a particularly simple form:
H(t)2 =
8πG
ε(t) .
3c2
(4.24)
Thus, for a given value of the Hubble parameter, there is a critical density,
εc (t) ≡
3c2
H(t)2 .
8πG
(4.25)
If the energy density ε(t) is greater than this value, the universe is positively
curved (κ = +1). If ε(t) is less than this value, the universe is negatively
curved (κ = −1). Since we know the current value of the Hubble parameter
to within 10%, we can compute the current value of the critical density to
within 20%:
εc,0 =
3c2 2
H = (8.3 ± 1.7) × 10−10 J m−3 = 5200 ± 1000 MeV m−3 . (4.26)
8πG 0
The critical density is frequently written as the equivalent mass density,
ρc,0 ≡ εc,0 /c2 = (9.2 ± 1.8) × 10−27 kg m−3 = (1.4 ± 0.3) × 1011 M¯ Mpc−3 .
(4.27)
Thus, the critical density is currently roughly equivalent to a density of one
hydrogen atom per 200 liters, or 140 solar mass stars per cubic kiloparsec.
This is definitely not a large density, by terrestrial standards. It’s not even a
large density by the standards of interstellar space within our Galaxy, where
even the hottest, most tenuous regions have a few protons per liter. However,
you must keep in mind that most of the volume of the universe consists of
intergalactic voids, where the density is extraordinarily low. When averaged
over scales of 100 Mpc or more, the mean density of the universe, as it turns
out, is close to the critical density.
In discussing the curvature of the universe, it is more convenient to use
not the absolute density ε, but the ratio of the density to the critical density
4.2. FLUID AND ACCELERATION EQUATIONS
65
εc . Thus, when talking about the energy density of the universe, cosmologists
often use the dimensionless density parameter
Ω(t) ≡
ε(t)
.
εc (t)
(4.28)
The most conservative limits on Ω – that is, limits that even the most belligerent cosmologist will hesitate to quarrel with – state that the current
value of the density parameter lies in the range 0.1 < Ω0 < 2.
In terms of the density parameter, the Friedmann equation can be written
in yet another form:
1 − Ω(t) = −
κc2
.
R02 a(t)2 H(t)2
(4.29)
Note that since the right hand side of equation (4.29) cannot change sign
as the universe expands, neither can the left hand side. If Ω < 1 at any
time, it remains less than one for all time; similarly, if Ω > 1 at any time,
it remains greater than one for all times, and if Ω = 1 at any time, Ω = 1
at all times. A leopard can’t change its spots; a universe governed by the
Friedmann equation can’t change the sign of its curvature. At the present
moment, the relation among curvature, density, and expansion rate can be
written in the form
κc2
1 − Ω0 = − 2 2 ,
(4.30)
R0 H 0
or
H02
κ
=
(Ω0 − 1) .
(4.31)
R02
c2
If you know Ω0 , you know the sign of the curvature (κ). If, in addition, you
know the Hubble distance, c/H0 , you can compute the radius of curvature
(R0 ).
4.2
The fluid and acceleration equations
Although the Friedmann equation is indeed important, it cannot, all by itself, tell us how the scale factor a(t) evolves with time. Even if we had
accurate boundary conditions (precise values for ε0 and H0 , for instance), it
still remains a single equation in two unknowns, a(t) and ε(t).5
5
Or, if we prefer, we may take the unknown functions as H(t) and Ω(t); in any case,
there are two of them.
66
CHAPTER 4. COSMIC DYNAMICS
We need another equation involving a and ε if we are to solve for a
and ε as functions of time. The Friedmann equation, in the Newtonian
approximation, is a statement of energy conservation; in particular, it says
that the sum of the gravitational potential energy and the kinetic energy
of expansion is constant. Energy conservation is a generally useful concept,
so let’s look at another manifestation of the same concept – the first law of
thermodynamics:
dQ = dE + P dV ,
(4.32)
where dQ is the heat flow into or out of a region, dE is the change in internal
energy, P is the pressure, and dV is the change in volume of the region.
This equation was applied in Section 2.5 to a comoving volume filled with
photons, but it applies equally well to a comoving volume filled with any
sort of fluid. If the universe is perfectly homogeneous, then for any volume
dQ = 0; that is, there is no bulk flow of heat. (Processes for which dQ = 0 are
known as adiabatic processes. Saying that the expansion of the universe is
adiabatic is also a statement about entropy. The change in entropy dS within
a region is given by the relation dS = dQ/T ; thus, an adiabatic process is
one in which entropy is not increased. A homogeneous, isotropic expansion
of the universe does not increase the universe’s entropy.) Since dQ = 0 for a
comoving volume as the universe expands, the first law of thermodynamics,
as applied to the expanding universe, reduces to the form
Ė + P V̇ = 0 .
(4.33)
For concreteness, consider a sphere of comoving radius rs expanding along
with the universal expansion, so that its proper radius is Rs (t) = a(t)rs . The
volume of the sphere is
4π 3
V (t) =
r a(t)3 ,
(4.34)
3 s
so the rate of change of the sphere’s volume is
4π 3 2
r (3a ȧ) = V
V̇ =
3 s
The internal energy of the sphere is
ȧ
3
a
µ
¶
.
(4.35)
E(t) = V (t)ε(t) ,
(4.36)
so the rate of change of the sphere’s internal energy is
Ė = V ε̇ + V̇ ε = V
ȧ
ε̇ + 3 ε
a
µ
¶
.
(4.37)
67
4.2. FLUID AND ACCELERATION EQUATIONS
Combining equations (4.33), (4.35), and (4.37), we find that the first law of
thermodynamics in an expanding (or contracting) universe takes the form
ȧ
ȧ
V ε̇ + 3 ε + 3 P
a
a
µ
¶
=0,
(4.38)
or
ȧ
(4.39)
ε̇ + 3 (ε + P ) = 0 .
a
The above equation is called the fluid equation, and is the second of the key
equations describing the expansion of the universe.6
The Friedmann equation and fluid equation are statements about energy
conservation. By combining the two, we can derive an acceleration equation
which tells how the expansion of the universe speeds up or slows down with
time. The Friedmann equation (equation 4.13), multiplied by a2 , takes the
form
8πG 2 κc2
ȧ2 =
(4.40)
εa − 2 .
3c2
R0
Taking the time derivative yields
2ȧä =
8πG 2
(ε̇a + 2εaȧ) .
3c2
(4.41)
Dividing by 2ȧa tells us
4πG a
ä
=
ε̇ + 2ε
a
3c2
ȧ
µ
¶
.
(4.42)
Using the fluid equation (equation 4.39), we may make the substitution
a
ε̇ = −3(ε + P )
ȧ
(4.43)
to find the usual form of the acceleration equation,
4πG
ä
= − 2 (ε + 3P ) .
a
3c
(4.44)
Note that if the energy density ε is positive, it provides a negative acceleration
– that is, it decreases the value of ȧ and reduces the relative velocity of
6
Write it on your forehead just underneath the Friedmann equation.
68
CHAPTER 4. COSMIC DYNAMICS
any two points in the universe. The acceleration equation also includes the
pressure P which is associated with the material filling the universe.7
A gas made of ordinary baryonic matter has a positive pressure P , resulting from the random thermal motions of the molecules, atoms, or ions of
which the gas is made. A gas of photons also has a positive pressure, as does
a gas of neutrinos or WIMPs. The positive pressure associated with these
components of the universe will cause the expansion to slow down. Suppose,
though, that the universe had a component with a pressure
P < −ε/3 .
(4.45)
Inspection of the acceleration equation (equation 4.44) shows us that such
a component will cause the expansion of the universe to speed up rather
than slow down. A negative pressure (also called “tension”) is certainly
permissible by the laws of physics. Compress a piece of rubber, and its
internal pressure will be positive; stretch the same piece of rubber, and its
pressure will be negative. In cosmology, the much-discussed cosmological
constant is a component of the universe with negative pressure. As we’ll
discuss in more detail in Section 4.4, a cosmological constant has P = −ε,
and thus causes a positive acceleration for the expansion of the universe.
4.3
Equations of state
To recap, we now have three key equations which describe how the universe
expands. There’s the Friedmann equation,
µ ¶2
ȧ
a
=
κc2
8πG
,
ε
−
3c2
R02 a2
(4.46)
the fluid equation,
ȧ
ε̇ + 3 (ε + P ) = 0 ,
a
and the acceleration equation,
ä
4πG
= − 2 (ε + 3P ) .
a
3c
7
(4.47)
(4.48)
Although we think of ε as an energy per unit volume and P as a force per unit area,
they both have the same dimensionality: in SI units, 1 J m−3 = 1 N m−2 = 1 kg m−1 s−2 .
69
4.3. EQUATIONS OF STATE
Of the above three equations, only two are independent, since equation (4.48),
as we’ve just seen, can be derived from equations (4.46) and (4.47). Thus,
we have a system of two independent equations in three unknowns – the
functions a(t), ε(t), and P (t). To solve for the scale factor, energy density,
and pressure as a function of cosmic time, we need another equation. What
we need is an equation of state; that is, a mathematical relation between the
pressure and energy density of the stuff that fills up the universe. If only we
had a relation of the form
P = P (ε) ,
(4.49)
life would be complete – or at least, our set of equations would be complete.
We could then, given the appropriate boundary conditions, solve them to find
how the universe expanded in the past, and how it will expand (or contract)
in the future.
In general, equations of state can be dauntingly complicated. Condensed
matter physicists frequently deal with substances in which the pressure is a
complicated nonlinear function of the density. Fortunately, cosmology usually deals with dilute gases, for which the equation of state is simple. For
substances of cosmological importance, the equation of state can be written
in a simple linear form:
P = wε ,
(4.50)
where w is a dimensionless number.
Consider, for instance, a low-density gas of non-relativistic massive particles. Non-relativistic, in this case, means that the random thermal motions
of the gas particles have peculiar velocities which are tiny compared to the
speed of light. Such a non-relativistic gas obeys the perfect gas law,
ρ
(4.51)
P = kT ,
µ
where µ is the mean mass of the gas particles. The energy density ε of a nonrelativistic gas is almost entirely contributed by the mass of the gas particles:
ε ≈ ρc2 . Thus, in terms of ε, the perfect gas law is
P ≈
kT
ε.
µc2
(4.52)
For a non-relativistic gas, the temperature T and the root mean square thermal velocity hv 2 i are associated by the relation
3kT = µhv 2 i .
(4.53)
70
CHAPTER 4. COSMIC DYNAMICS
Thus, the equation of state for a non-relativistic gas can be written in the
form
Pnon−rel = wεnon−rel ,
(4.54)
where
hv 2 i
¿1.
(4.55)
3c2
Most of the gases we encounter in everyday life are non-relativistic. For instance, in air at room temperature, the nitrogen molecules are slow-poking
along with a root mean square velocity of ∼ 500 m s−1 , yielding w ∼ 10−12 .
Even in astronomical contexts, gases are mainly non-relativistic at the present
moment. Within a gas of ionized hydrogen, for instance, the electrons are
non-relativistic as long as T ¿ 6 × 109 K; the protons are non-relativistic
when T ¿ 1013 K.
A gas of photons, or other massless particles, is guaranteed to be relativistic. Although photons have no mass, they have momentum, and hence
exert pressure. The equation of state of photons, or of any other relativistic
gas, is
1
(4.56)
Prel = εrel .
3
(This relation has already been used in Section 2.5, to compute how the
Cosmic Microwave Background cools as the universe expands.) A gas of
highly relativistic massive particles (with hv 2 i ∼ c2 ) will also have w = 1/3; a
gas of mildly relativistic particles (with 0 < hv 2 i < c2 ) will have 0 < w < 1/3.
The equation-of-state parameter w can’t take on arbitrary values. Small
perturbations in a substance with pressure P will travel at the speed of sound.
For adiabatic perturbations in a gas with pressure P and energy density ε,
the sound speed is given by the relation
w≈
c2s
=c
2
Ã
dP
dε
!
.
(4.57)
√
In a substance with w > 0, the sound speed is thus cs = wc.8 Sound waves
cannot travel faster than the speed of light; if they did, you would be able to
send a sound signal into the past, and violate causality. Thus, w is restricted
to values w ≤ 1.
8
In a substance with w < 0, the sound speed is an imaginary number; this implies that
small pressure perturbations to a substance with negative pressure will not constitute
stably propagating sound waves, but will have amplitudes which grow or decay with time.
4.4. LEARNING TO LOVE LAMBDA
71
Some values of w are of particular interest. For instance, the case w = 0
is of interest, because we know that our universe contains non-relativistic
matter. The case w = 1/3 is of interest, because we know that our universe
contains photons. For simplicity, I will refer to the component of the universe
which consists of non-relativistic particles (and hence has w ≈ 0) as “matter”.
I will refer to the component which consists of photons and other relativistic
particles (and hence has w = 1/3) as “radiation”. The case w < −1/3 is
of interest, because a component with w < −1/3 will provide a positive
acceleration (ä > 0 in equation 4.48). A component of the universe with
w < −1/3 is sometimes referred to generically as “dark energy” (a phrase
coined by the cosmologist Michael Turner). One form of “dark energy” is
of special interest; some observational evidence, which we’ll review in future
chapters, indicates that our universe may contain a cosmological constant. A
cosmological constant may be simply defined as a component of the universe
which has w = −1, and hence has P = −ε. The cosmological constant,
also designated by the Greek letter Λ, has had a controversial history, and is
still the subject of debate. To learn why cosmologists have had such a longstanding love/hate affair with the cosmological constant Λ, it is necessary to
make a brief historical review.
4.4
Learning to love lambda
The cosmological constant Λ was first introduced by Albert Einstein. After
publishing his first paper on general relativity in 1915, Einstein, naturally
enough, wanted to apply his field equation to the real universe. He looked
around, and noted that the universe contains both radiation and matter.
Since Einstein, along with every other earthling of his time, was unaware of
the existence of the Cosmic Microwave Background, he thought that most
of the radiation in the universe was in the form of starlight. He also noted,
quite correctly, that the energy density of starlight in our Galaxy is much
less than the rest energy density of the stars.9 Thus, Einstein concluded that
the primary contribution to the energy density of the universe was from nonrelativistic matter, and that he could safely make the approximation that we
live in a pressureless universe.
So far, Einstein was on the right track. However, in 1915, astronomers
were unaware of the existence of the expansion of the universe. In fact, it was
9
That is, stars have only converted a small fraction of their initial mass into light.
72
CHAPTER 4. COSMIC DYNAMICS
by no means settled that galaxies besides our own actually existed. After
all, the sky is full of faint fuzzy patches of light. It took some time to sort
out that some of the faint fuzzy patches are glowing clouds of gas within our
Galaxy and that some of them are galaxies in their own right, far beyond
our own Galaxy. Thus, when Einstein asked, “Is the universe expanding or
contracting?” he looked, not at the motions of galaxies, but at the motions of
stars within our Galaxy. Einstein noted that some stars are moving toward us
and that others are moving away from us, with no evidence that the Galaxy
is expanding or contracting.
The incomplete evidence available to Einstein led him to the belief that
the universe is static – neither expanding nor contracting – and that it has
a positive energy density but negligible pressure. Einstein then had to ask
the question, “Can a universe filled with non-relativistic matter, and nothing
else, be static?” The answer to this question is “No!” A universe containing
nothing but matter must, in general, be either expanding or contracting.
The reason why this is true can be illustrated in a Newtonian context. If the
mass density of the universe is ρ, then the gravitational potential Φ is given
by Poisson’s equation:
∇2 Φ = 4πGρ .
(4.58)
The gravitational acceleration ~a at any point in space is then found by taking
the gradient of the potential:
~ .
~a = −∇Φ
(4.59)
In a static universe, ~a must vanish everywhere in space. Thus, the potential Φ must be constant in space. However, if Φ is constant, then (from
equation 4.58)
1
∇2 Φ = 0 .
(4.60)
ρ=
4πG
The only permissible static universe, in this analysis, is a totally empty universe. If you create a matter-filled universe which is initially static, then
gravity will cause it to contract. If you create a matter-filled universe which
is initially expanding, then it will either expand forever (if the Newtonian energy U is greater than or equal to zero) or reach a maximum radius and then
collapse (if U < 0). Trying to make a matter-filled universe which doesn’t
expand or collapse is like throwing a ball into the air and expecting it to
hover there.
73
4.4. LEARNING TO LOVE LAMBDA
How did Einstein surmount this problem? How did he reconcile the fact
that the universe contains matter with his desire for a static universe? Basically, he added a fudge factor to the equations. In Newtonian terms, what
he did was analogous to rewriting Poisson’s equation in the form
∇2 Φ + Λ = 4πGρ .
(4.61)
The new term, symbolized by the Greek letter Λ, came to be known as the
cosmological constant. Note that it has dimensionality (time)−2 . Introducing
Λ into Poisson’s equation allows the universe to be static if you set Λ = 4πGρ.
In general relativistic terms, what Einstein did was to add an additional
term, involving Λ, to his field equation (the relativistic equivalent of Poisson’s equation). If the Friedmann equation is re-derived from Einstein’s field
equation, with the Λ term added, it becomes
µ ¶2
ȧ
a
=
8πG
κc2
Λ
ε
−
+
.
3c2
R02 a2
3
(4.62)
The fluid equation is unaffected by the presence of a Λ term, so it still has
the form
ȧ
ε̇ + 3 (ε + P ) = 0 .
(4.63)
a
With the Λ term present, the acceleration equation becomes
Λ
ä
4πG
= − 2 (ε + 3P ) + .
a
3c
3
(4.64)
A look at the Friedmann equation (4.62) tells us that adding the Λ term is
equivalent to adding a new component to the universe with energy density
εΛ ≡
c2
Λ.
8πG
(4.65)
If Λ remains constant with time, then so does its associated energy density
εΛ . The fluid equation (4.63) tells us that to have εΛ constant with time, the
Λ term must have an associated pressure
PΛ = −εΛ = −
c2
Λ.
8πG
(4.66)
Thus, we can think of the cosmological constant as a component of the universe which has a constant density εΛ and a constant pressure PΛ = −εΛ .
74
CHAPTER 4. COSMIC DYNAMICS
By introducing a Λ term into his equations, Einstein got the static model
universe he wanted. For the universe to remain static, both ȧ and ä must
be equal to zero. If ä = 0, then in a universe with matter density ρ and
cosmological constant Λ, the acceleration equation (4.64) reduces to
0=−
Λ
4πG
ρ+ .
3
3
(4.67)
Thus, Einstein had to set Λ = 4πGρ in order to produce a static universe,
just as in the Newtonian case. If ȧ = 0, the Friedmann equation (4.62)
reduces to
8πG
κc2 Λ
κc2
0=
(4.68)
ρ − 2 + = 4πGρ − 2 .
3
R0
3
R0
Einstein’s static model therefore had to be positively curved (κ = +1), with
a radius of curvature
c
c
R0 =
= 1/2 .
(4.69)
1/2
2(πGρ)
Λ
Although Einstein published the details of his static, positively curved, matterfilled model in the spring of 1917, he was dissatisfied with the model. He
believed that the cosmological constant was “gravely detrimental to the formal beauty of the theory”. In addition to its aesthetic shortcomings, the
model had a practical defect; it was unstable. Although Einstein’s static
model was in equilibrium, with the repulsive force of Λ balancing the attractive force of ρ, it was an unstable equilibrium. Consider expanding Einstein’s
universe just a tiny bit. The energy density of Λ remains unchanged, but
the energy density of matter drops. Thus, the repulsive force is greater than
the attractive force, and the universe expands further. This causes the matter density to drop further, which causes the expansion to accelerate, which
causes the matter density to drop further, and so forth. Expanding Einstein’s
static universe triggers runaway expansion; similarly, compressing it causes
a runaway collapse.
Einstein was willing, even eager, to dispose of the “ugly” cosmological
constant in his equations. Hubble’s 1929 paper on the redshift – distance relation gave Einstein the necessary excuse for tossing Λ onto the rubbish heap.
(Einstein later described the cosmological constant Λ as “the greatest blunder of my career.”) Ironically, however, the same paper that caused Einstein
to abandon the cosmological constant caused other scientists to embrace it.
In his initial analysis, remember, Hubble badly underestimated the distance
4.4. LEARNING TO LOVE LAMBDA
75
to galaxies, and hence overestimated the Hubble constant. Hubble’s initial
value of H0 = 500 km s−1 Mpc−1 leads to a Hubble time of H0−1 = 2 Gyr,
less than half the age of the Earth, as known from radioactive dating. How
could cosmologists reconcile a short Hubble time with an old Earth? Some
cosmologists pointed out that one way to increase the age of the universe for
a given value of H0−1 was to introduce a cosmological constant. If the value
of Λ is large enough to make ä > 0, then ȧ was smaller in the past than it is
now, and consequently the universe is older than H0−1 .
If Λ has a value greater than 4πGρ0 , then the expansion of the universe is
accelerating, and the universe can be arbitrarily old for a given value of H0−1 .
Since 1917, the cosmological constant has gone in and out of fashion, like
sideburns or short skirts. It has been particularly fashionable during periods
when the favored value of the Hubble time H0−1 has been embarrassingly
short compared to the estimated ages of astronomical objects. Currently,
the cosmological constant is popular, thanks to observations, which we will
discuss in Chapter 7, that indicate that the expansion of the universe does,
indeed, have a positive acceleration.
A question which has been asked since the time of Einstein – and one
which I’ve assiduously dodged until this moment – is “What is the physical
cause of the cosmological constant?” In order to give Λ a real physical
meaning, we need to identify some component of the universe whose energy
density εΛ remains constant as the universe expands or contracts. Currently,
the leading candidate for this component is the vacuum energy.
In classical physics, the idea of a vacuum having energy is nonsense. A
vacuum, from the classical viewpoint, contains nothing; and as King Lear
would say, “Nothing can come of nothing.” In quantum physics, however, a
vacuum is not a sterile void. The Heisenberg uncertainty principle permits
particle-antiparticle pairs to spontaneously appear and then annihilate in an
otherwise empty vacuum. The total energy ∆E and the lifetime ∆t of these
pairs of virtual particles must satisfy the relation10
∆E∆t ≤ h .
(4.70)
Just as there’s an energy density associated with the real particles in the
universe, there is an energy density εvac associated with the virtual particleantiparticle pairs. The vacuum density εvac is a quantum phenomenon which
10
The usual analogy that’s made is to an embezzling bank teller who takes money from
the till but who always replaces it before the auditor comes around. Naturally, the more
money a teller is entrusted with, the more frequently the auditor checks up on her.
76
CHAPTER 4. COSMIC DYNAMICS
doesn’t give a hoot about the expansion of the universe and is independent
of time as the universe expands or contracts.
Unfortunately, computing the numerical value of εvac is an exercise in
quantum field theory which has not yet been successfully completed. It has
been suggested that the natural value for the vacuum energy density is the
Planck energy density,
εvac ∼
EP
`3P
(???) .
(4.71)
As we’ve seen in Chapter 1, the Planck energy is large by particle physics
standards (EP = 1.2×1028 eV), while the Planck length is small by anybody’s
standards (`P = 1.6 × 10−35 m). This gives an energy density
εvac ∼ 3 × 10133 eV m−3
(!!!) .
(4.72)
This is 124 orders of magnitude larger than the current critical density for
our universe, and represents a spectacularly bad match between theory and
observations. Obviously, we don’t know much yet about the energy density
of the vacuum! This is a situation where astronomers can help particle
physicists, by deducing the value of εΛ from observations of the expansion
of the universe. By looking at the universe at extremely large scales, we are
indirectly examining the structure of the vacuum on extremely small scales.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Bernstein (1995), ch. 2: Friedmann equation, with applications
Harrison (2000), ch. 16: Newtonian derivation of Friedmann equation
Liddle (1999), ch. 3: Derivation of Friedmann, fluid, and acceleration equations
Linder (1997), ch. 2: Dynamics of the universe, including a discussion of
equations of state
Rich (2001), ch. 4: “Relativistically correct” derivation of the Friedmann
equation
4.4. LEARNING TO LOVE LAMBDA
77
Problems
(4.1) Suppose the energy density of the cosmological constant is equal to the
present critical density εΛ = εc,0 = 5200 MeV m−3 . What is the total
energy of the cosmological constant within a sphere 1 AU in radius?
What is the rest energy of the Sun (E¯ = M¯ c2 )? Comparing these two
numbers, do you expect the cosmological constant to have a significant
effect on the motion of planets within the Solar System?
(4.2) Consider Einstein’s static universe, in which the attractive force of
the matter density ρ is exactly balanced by the repulsive force of the
cosmological constant, Λ = 4πGρ. Suppose that some of the matter
is converted into radiation (by stars, for instance). Will the universe
start to expand or contract? Explain your answer.
(4.3) If ρ = 3×10−27 kg m−3 , what is the radius of curvature R0 of Einstein’s
static universe? How long would it take a photon to circumnavigate
such a universe?
(4.4) Suppose that the universe were full of regulation baseballs, each of
mass mbb = 0.145 kg and radius rbb = 0.0369 m. If the baseballs were
distributed uniformly throughout the universe, what number density
of baseballs would be required to make the density equal to the critical
density? (Assume non-relativistic baseballs.) Given this density of
baseballs, how far would you be able to see, on average, before your line
of sight intersected a baseball? In fact, we can see galaxies at a distance
∼ c/H0 ∼ 4000 Mpc; does the transparency of the universe on this
length scale place useful limits on the number density of intergalactic
baseballs? [Note to readers outside North America or Japan: feel free
to substitute regulation cricket balls, with mcr = 0.160 kg and rcr =
0.0360 m.]
(4.5) The principle of wave-particle duality tells us that a particle with
momentum p has an associated de Broglie wavelength of λ = h/p; this
wavelength increases as λ ∝ a as the universe expands. The total
energy density of a gas of particles can be written as ε = nE, where n
is the number density of particles, and E is the energy per particle. For
simplicity, let’s assume that all the gas particles have the same mass
78
CHAPTER 4. COSMIC DYNAMICS
m and momentum p. The energy per particle is then simply
E = (m2 c4 + p2 c2 )1/2 = (m2 c4 + h2 c2 /λ2 )1/2 .
(4.73)
Compute the equation-of-state parameter w for this gas as a function
of the scale factor a. Show that w = 1/3 in the highly relativistic limit
(a → 0, p → ∞) and that w = 0 in the highly non-relativistic limit
(a → ∞, p → 0).
Chapter 5
Single-Component Universes
In a spatially homogeneous and isotropic universe, the relation among the
energy density ε(t), the pressure P (t), and the scale factor a(t) is given by
the Friedmann equation,
µ ¶2
ȧ
a
the fluid equation,
8πG
κc2
=
ε− 2 2 ,
3c2
R0 a
(5.1)
ȧ
ε̇ + 3 (ε + P ) = 0 ,
a
(5.2)
P = wε .
(5.3)
and the equation of state,
In principle, given the appropriate boundary conditions, we can solve equations (5.1), (5.2), and (5.3) to yield ε(t), P (t), and a(t) for all times, past
and future.
5.1
Evolution of energy density
In reality, the evolution of our universe is complicated by the fact that it
contains different components with different equations of state. We know
that the universe contains non-relativistic matter and radiation – that’s a
conclusion as firm as the earth under your feet and as plain as daylight. Thus,
the universe contains components with both w = 0 and w = 1/3. It may
well contain a cosmological constant, with w = −1. Moreover, the possibility
exists that it may contain still more exotic components, with different values
79
80
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
of w. Fortunately for the cause of simplicity, the energy density and pressure
for the different components of the universe are additive. We may write
the total energy density ε as the sum of the energy density of the different
components:
X
ε=
εw ,
(5.4)
w
where εw represents the energy density of the component with equation-ofstate parameter w. The total pressure P is the sum of the pressures of the
different components:
X
X
P =
Pw =
wεw .
(5.5)
w
w
Because the energy densities and pressures add in this way, the fluid equation
(5.2) must hold for each component separately, as long as there is no interaction between the different components. If this is so, then the component
with equation-of-state parameter w obeys the equation
ȧ
ε̇w + 3 (εw + Pw ) = 0
a
(5.6)
or
ȧ
ε̇w + 3 (1 + w)εw = 0 .
a
Equation (5.7) can be rearranged to yield
da
dεw
= −3(1 + w)
.
εw
a
(5.7)
(5.8)
If we assume that w is constant, then
εw (a) = εw,0 a−3(1+w) .
(5.9)
Here, I’ve used the usual normalization that a0 = 1 at the present day, when
the energy density of the w component is εw,0 . Note that equation (5.9) is derived solely from the fluid equation and the equation of state; the Friedmann
equation doesn’t enter into it.
Starting from the general result of equation (5.9), we conclude that the
energy density εm associated with non-relativistic matter decreases as the
universe expands with the dependence
εm (a) = εm,0 /a3 .
(5.10)
5.1. EVOLUTION OF ENERGY DENSITY
81
Figure 5.1: The dilution of non-relativistic particles (“matter”) and relativistic particles (“radiation”) as the universe expands.
The energy density in radiation, εr , drops at the steeper rate
εr (a) = εr,0 /a4 .
(5.11)
Why this difference between matter and radiation? We may write the energy density of either component in the form ε = nE, where n is the number
density of particles and E is the mean energy per particle. For both relativistic and non-relativistic particles, the number density has the dependence
n ∝ a−3 as the universe expands, assuming that particles are neither created
nor destroyed.
Consider, once again, a sphere of comoving radius rs , which expands
along with the general expansion of the universe (Figure 5.1). When its
82
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
proper radius expands by a factor 2, its volume increases by a factor 8, and
the density of particles which it contains falls to 1/8 its previous value. The
energy of non-relativistic particles (shown in the upper panel of Figure 5.1) is
contributed solely by their rest mass (E = mc2 ) and remains constant as the
universe expands. Thus, for non-relativistic matter, εm = nE = n(mc2 ) ∝
a−3 . The energy of photons or other massless particles (shown in the lower
panel of Figure 5.1) has the dependence E = hc/λ ∝ a−1 , since, as shown in
equation (3.45), λ ∝ a as the universe expands. Thus, for photons and other
massless particles, εr = nE = n(hc/λ) ∝ a−3 a−1 ∝ a−4 .
Although I’ve explained why photons have an energy density εr ∝ a−4 ,
the explanation required the assumption that photons are neither created
nor destroyed. Such an assumption is not, strictly speaking, correct; photons are continuously being created. The Sun, for instance, is emitting out
roughly 1045 photons every second. However, so numerous are the photons
of the Cosmic Microwave Background, it turns that the energy density of
the CMB is larger than the energy density of all the photons emitted by all
the stars in the history of the universe. To see why this is true, remember,
from Section 2.5, that the present energy density of the CMB, which has a
temperature T0 = 2.725 K, is
εCMB,0 = αT04 = 4.17 × 10−14 J m−3 = 0.260 MeV m−3 .
(5.12)
yielding a density parameter for the CMB of
ΩCMB,0 ≡
εCMB,0
0.260 MeV m−3
=
= 5.0 × 10−5 .
εc,0
5200 MeV m−3
(5.13)
Although the energy density of the CMB is small compared to the critical
density, it is large compared to the energy density of starlight. Remember
from Section 2.3 that the present luminosity density of galaxies is
nL ≈ 2 × 108 L¯ Mpc−3 ≈ 2.6 × 10−33 watts m−3 .
(5.14)
As a very rough estimate, let’s assume that galaxies have been emitting
light at this rate for the entire age of the universe, t0 ≈ H0−1 ≈ 14 Gyr ≈
4.4 × 1017 s. This gives an energy density in starlight of
εstarlight,0 ∼ nLt0 ∼ (2.6 × 10−33 J s−1 m−3 )(4.4 × 1017 s)
∼ 1 × 10−15 J m−3 ∼ 0.007 MeV m−3 .
(5.15)
5.1. EVOLUTION OF ENERGY DENSITY
83
Thus, the average energy density of starlight is currently only ∼ 3% of the
energy density of the CMB. In fact, the estimate given above is a very rough
one indeed. Measurements of background radiation from ultraviolet wavelengths to the near infrared, which includes both direct starlight and starlight
absorbed and reradiated by dust, yield the larger value εstarlight /εCMB ≈ 0.1.
In the past, the ratio of starlight density to CMB density was even smaller
than it is today. For most purposes, it is an acceptable approximation to ignore non-CMB photons when computing the mean energy density of photons
in the universe.
The Cosmic Microwave Background, remember, is a relic of the time
when the universe was hot and dense enough to be opaque to photons. If we
extrapolate further back in time, we reach a time when the universe was hot
and dense enough to be opaque to neutrinos. As a consequence, there should
be a Cosmic Neutrino Background today, analogous to the Cosmic Microwave
Background. The energy density in neutrinos should be comparable to, but
not exactly equal to, the energy density in photons. A detailed calculation
indicates that the energy density of each neutrino flavor should be
ε=
7 4
8 11
µ
¶4/3
εCMB ≈ 0.227 εCMB .
(5.16)
(The above result assumes that the neutrinos are relativistic, or, equivalently,
that their energy is much greater than their rest energy mν c2 .) The density
parameter of the Cosmic Neutrino Background, taking into account all three
flavors of neutrino, should then be
Ων = 0.681 ΩCMB ,
(5.17)
as long as all neutrino flavors are relativistic. The mean energy per neutrino
will be comparable to, but not exactly equal to, the mean energy per photon:
Eν ≈
5 × 10−4 eV
,
a
(5.18)
as long as Eν > mν c2 . When the mean energy of a particular neutrino species
drops to ∼ mν c2 , then it makes the transition from being “radiation” to being
“matter”.
The neutrinos of the Cosmic Neutrino Background, I should note, have
not yet been detected. Although neutrinos have been detected from the Sun
and from Supernova 1987A, current technology permits the detection only
84
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
of neutrinos with energy E > 0.1MeV, far more energetic than the neutrinos
of the Cosmic Neutrino Background.
If all neutrino species are effectively massless today, with mν c2 ¿ 5 ×
−4
10 eV, then the present density parameter in radiation is
Ωr,0 = ΩCMB,0 + Ων,0 = 5.0 × 10−5 + 3.4 × 10−5 = 8.4 × 10−5 .
(5.19)
We know the energy density of the Cosmic Microwave Background with high
precision. We can calculate theoretically what the energy density of the Cosmic Neutrino Background should be. Unfortunately, the total energy density
of non-relativistic matter is poorly known, as is the energy density of the
cosmological constant. As we shall see in future chapters, the available evidence favors a universe in which the density parameter for matter is currently
Ωm,0 ∼ 0.3, while the density parameter for the cosmological constant is currently ΩΛ,0 ∼ 0.7. Thus, when I want to employ a model which matches the
observed properties of the real universe, I will use what I call the “Benchmark Model”; this model has Ωr,0 = 8.4 × 10−5 in radiation, Ωm,0 = 0.3 in
non-relativistic matter, and ΩΛ,0 = 1 − Ωr,0 − Ωm,0 ≈ 0.7 in a cosmological
constant.1
In the Benchmark Model, at the present moment, the ratio of the energy
density in Λ to the energy density in matter is
εΛ,0
ΩΛ,0
0.7
≈ 2.3 .
=
≈
εm,0
Ωm,0
0.3
(5.20)
In the language of cosmologists, the cosmological constant is “dominant”
over matter today in the Benchmark Model. In the past, however, when the
scale factor was smaller, the ratio of densities was
εΛ (a)
εΛ,0
εΛ,0 3
=
=
a .
3
εm (a)
εm,0 /a
εm,0
(5.21)
If the universe has been expanding from an initial very dense state, at some
moment in the past, the energy density of matter and Λ must have been
equal. This moment of matter-Λ equality occurred when the scale factor was
amΛ =
1
Ã
Ωm,0
ΩΛ,0
!1/3
≈
µ
0.3
0.7
¶1/3
Note that the Benchmark Model is spatially flat.
≈ 0.75 .
(5.22)
5.1. EVOLUTION OF ENERGY DENSITY
85
Similarly, the ratio of the energy density in matter to the energy density in
radiation is currently
Ωm,0
0.3
εm,0
=
≈
≈ 3600
εr,0
Ωr,0
8.4 × 10−5
(5.23)
if all three neutrino flavors in the Cosmic Neutrino Background are still
relativistic today; it’s even larger if some or all of the neutrino flavors are
currently non-relativistic. Thus, matter is now strongly dominant over radiation. However, in the past, the ratio of matter density to energy density
was
εm (a)
εm,0
=
a.
(5.24)
εr (a)
εr,0
Thus, the moment of radiation-matter equality took place when the scale
factor was
1
εm,0
≈
≈ 2.8 × 10−4 .
(5.25)
arm =
εr,0
3600
Note that as long as a neutrino’s mass is mν c2 ¿ (3600)(5×10−4 eV) ∼ 2 eV,
then it would have been relativistic at a scale factor a = 1/3600, and hence
would have been “radiation” then even if it is “matter” today.
To generalize, if the universe contains different components with different
values of w, equation (5.9) tells us that in the limit a → 0, the component
with the largest value of w is dominant. If the universe expands forever,
then as a → ∞, the component with the smallest value of w is dominant.
The evidence indicates we live in a universe where radiation (w = 1/3) was
dominant during the early stages, followed by a period when matter (w = 0)
was dominant. If the presently available evidence is correct, and we live in a
universe described by the Benchmark Model, we have only recently entered
a period when the cosmological constant Λ (w = −1) is dominant.
In a universe which is continuously expanding, the scale factor a is a
monotonically increasing function of t. Thus, in a continuously expanding
universe, the scale factor a can be used as a surrogate for the cosmic time
t. We can refer, for instance, to the moment when a = 2.8 × 10−4 with the
assurance that we are referring to a unique moment in the history of the
universe. In addition, because of the simple relation between scale factor
and redshift,
1
a=
,
(5.26)
1+z
86
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
cosmologists often use redshift as a surrogate for time. For example, they
make statements such as, “Radiation-matter equality took place at a redshift
zrm ≈ 3600.” That is, light that was emitted at the time of radiation-matter
equality is observed by us with its wavelength increased by a factor of 3600.
One reason why cosmologists use scale factor or redshift as a surrogate for
time is that the conversion from a to t is not simple to calculate in a multiplecomponent universe like our own. In a universe with many components, can
be written in the form
κc2
8πG X
−1−3w
εw,0 a
− 2 .
ȧ =
3c2 w
R0
2
(5.27)
Each term on the right hand side of equation (5.27) has a different dependence on scale factor; radiation contributes a term ∝ a−2 , matter contributes
a term ∝ a−1 , curvature contributes a term independent of a, and the cosmological constant Λ contributes a term ∝ a2 . Solving equation (5.27)
for a multiple-component model, such as the Benchmark Model, does not
yield a simple analytic form for a(t). However, looking at simplified singlecomponent universes, in which there is only one term on the right hand side
of equation (5.27), yields useful insight into the physics of an expanding
universe.
5.2
Curvature only
A particularly simple universe is one which is empty – no radiation, no matter, no cosmological constant, no contribution to ε of any sort. For this
universe, the Friedmann equation (5.27) takes the form
ȧ2 = −
κc2
.
R02
(5.28)
One solution to this equation has ȧ = 0 and κ = 0. An empty, static,
spatially flat universe is a permissible solution to the Friedmann equation.
This is the universe whose geometry is described by the Minkowski metric
of equation (3.20), and in which all the transformations of special relativity
hold true.
However, equation (5.28) tells us that it is also possible to have an empty
universe with κ = −1. (Positively curved empty universes are forbidden,
87
5.2. CURVATURE ONLY
since that would require an imaginary value of ȧ in equation 5.28.) A negatively curved empty universe must be expanding or contracting, with
ȧ = ±
c
.
R0
(5.29)
In an expanding empty universe, integration of this relation yields a scale
factor of the form
t
a(t) =
,
(5.30)
t0
where t0 = R0 /c.2 In Newtonian terms, if there’s no gravitational force at
work, then the relative velocity of any two points is constant, and thus the
scale factor a simply increases linearly with time in an empty universe. The
scale factor in an empty, expanding universe is shown as the dashed line in
Figure 5.2. Note that in an empty universe, t0 = H0−1 ; with nothing to speed
or slow the expansion, the age of the universe is exactly equal to the Hubble
time.
An empty, expanding universe might seem nothing more than a mathematical curiosity.3 However, if a universe has a density ε which is very small
compared to the critical density εc (that is, if Ω ¿ 1), then the linear scale
factor of equation (5.30) is a good approximation to the true scale factor.
Suppose you were in an expanding universe with a negligibly small value
for the density parameter Ω, so that you could reasonably approximate it as
an empty, negatively curved universe, with t0 = H0−1 = R0 /c. You observe
a distant light source, such as a galaxy, which has a redshift z. The light
which you observe now, at t = t0 , was emitted at an earlier time, t = te . In
an empty expanding universe,
1+z =
t0
1
=
,
a(te )
te
(5.31)
so it is easy to compute the time when the light you observe from the source
was emitted:
H −1
t0
= 0 .
(5.32)
te =
1+z
1+z
When observing a galaxy with a redshift z, in addition to asking “When
was the light from that galaxy emitted?” you may also ask “How far away
2
Such an empty, negatively curved, expanding universe is sometimes called a Milne
universe, after cosmologist to study its properties.
3
If a universe contains nothing, there will be no observers in it to detect the expansion.
88
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
8
Λ
a
6
empty
matter
4
radiation
2
0
−2
0
2
4
H0(t−t0)
6
8
Figure 5.2: The scale factor as a function of time for an expanding, empty universe (dashed), a flat, matter-dominated universe (dotted), a flat, radiationdominated universe (solid), and a flat, Λ-dominated universe (dot-dash).
89
5.2. CURVATURE ONLY
is that galaxy?” In Section 3.3 we saw that in any universe described by a
Robertson-Walker metric, the current proper distance from an observer at
the origin to a galaxy at coordinate location (r, θ, φ) is (see equation 3.28)
dp (t0 ) = a(t0 )
Z
r
0
dr = r .
(5.33)
Moreover, if light is emitted by the galaxy at time te and detected by the
observer at time t0 , the null geodesic followed by the light satisfies equation (3.39):
Z r
Z t0
dt
=
c
dr = r .
(5.34)
te a(t)
0
Thus, the current proper distance from you (the observer) to the galaxy (the
light source) is
Z t0
dt
dp (t0 ) = c
.
(5.35)
te a(t)
Equation (5.35) holds true in any universe whose geometry is described by
a Robertson-Walker metric. In the specific case of an empty expanding universe, a(t) = t/t0 , and thus
dp (t0 ) = ct0
Z
t0
te
dt
= ct0 ln(t0 /te ) .
t
(5.36)
Expressed in terms of the redshift z of the observed galaxy,
dp (t0 ) =
c
ln(1 + z) .
H0
(5.37)
This relation is plotted as the dashed line in the upper panel of Figure 5.3.
In the limit z ¿ 1, there is a linear relation between dp and z, as seen
observationally in Hubble’s law. In the limit z À 1, however, dp ∝ ln z in an
empty expanding universe.
In an empty expanding universe, we can see objects which are currently
at an arbitrarily large distance. However, at distances dp (t0 ) À c/H0 , the
redshift increases exponentially with distance. At first glance, it may seem
counterintuitive that you can see a light source at a proper distance much
greater than c/H0 when the age of the universe is only 1/H0 . However, you
must remember that dp (t0 ) is the proper distance to the light source at the
time of observation; at the time of emission, the proper distance dp (te ) was
90
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
(H0/c) dp(t0)
100
Observation
10
1
.1
.01
.01
(H0/c) dp(te)
100
.1
1
z
10
100 1000
10
100 1000
Emission
10
1
.1
.01
.01
.1
1
z
Figure 5.3: The proper distance to an object with observed redshift z. The
upper panel shows the proper distance at the time the light is observed; the
lower panel shows the proper distance at the time the light was emitted. The
line types are the same as those of Figure 5.2.
5.3. SPATIALLY FLAT UNIVERSES
91
smaller by a factor a(te )/a(t0 ) = 1/(1 + z). In an empty expanding universe,
the proper distance at the time of emission was
dp (te ) =
c ln(1 + z)
,
H0 1 + z
(5.38)
shown as the dashed line in the lower panel of Figure 5.3. In an empty
expanding universe, dp (te ) has a maximum for objects with a redshift z =
e − 1 ≈ 1.7, where dp (te ) = (1/e)(c/H0 ) ≈ 0.37(c/H0 ). Objects with much
higher redshifts are seen as they were very early in the history of the universe,
when their proper distance from the observer was very small.
5.3
Spatially flat universes
Setting the energy density ε equal to zero is one way of simplifying the
Friedmann equation. Another way is to set κ = 0 and to demand that the
universe contain only a single component, with a single value of w. In such a
spatially flat, single-component universe, the Friedmann equation takes the
simple form
8πGε0 −(1+3w)
ȧ2 =
a
.
(5.39)
3c2
To solve this equation, we first make the educated guess that the scale factor
has the power law form a ∝ tq . The left hand side of equation (5.39) is then
∝ t2q−2 , and the right hand side is ∝ t−(1+3w)q , yielding the solution
q=
2
,
3 + 3w
(5.40)
with the restriction w 6= −1. With the proper normalization, the scale factor
in a spatially flat, single-component universe is
a(t) = (t/t0 )2/(3+3w) ,
(5.41)
where the age of the universe, t0 , is linked to the present energy density by
the relation
Ã
!1/2
1
c2
t0 =
.
(5.42)
1 + w 6πGε0
The Hubble constant in such a universe is
µ ¶
2
ȧ
t−1
.
=
H0 ≡
a t=t0
3(1 + w) 0
(5.43)
92
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
The age of the universe, in terms of the Hubble time, is then
t0 =
2
H −1 .
3(1 + w) 0
(5.44)
In a spatially flat universe, if w > −1/3, the universe is younger than the
Hubble time. If w < −1/3, the universe is older than the Hubble time.
As a function of scale factor, the energy density of a component with
equation-of-state parameter w is
ε(a) = ε0 a−3(1+w) ,
(5.45)
so in a spatially flat universe with only a single component, the energy density
as a function of time is (combining equations 5.41 and 5.45)
ε(t) = ε0 (t/t0 )−2 ,
(5.46)
regardless of the value of w. Making the substitutions
3c2 2
H
8πG 0
(5.47)
2
H −1 ,
3(1 + w) 0
(5.48)
ε0 = εc,0 =
and
t0 =
equation (5.46) can be written in the form
ε(t) =
c2 −2
1
t .
6π(1 + w)2 G
(5.49)
Expressed in terms of Planck units (introduced in Chapter 1), this relation
between energy density and cosmic time is
ε(t) =
1
Ep
2
6π(1 + w) `3p
µ
t
tP
¶−2
.
(5.50)
Suppose yourself to be in a spatially flat, single-component universe. If
you see a galaxy, or other distant light source, with a redshift z, you can use
the relation
µ ¶2/(3+3w)
a(t0 )
t0
1+z =
=
(5.51)
a(te )
te
93
5.3. SPATIALLY FLAT UNIVERSES
to compute the time te at which the light from the distant galaxy was emitted:
te =
2
1
t0
=
.
3(1+w)/2
(1 + z)
3(1 + w)H0 (1 + z)3(1+w)/2
(5.52)
The current proper distance to the galaxy is
dp (t0 ) = c
Z
t0
te
dt
3(1 + w)
= ct0
[1 − (te /t0 )(1+3w)/(3+3w) ] ,
a(t)
1 + 3w
(5.53)
when w 6= −1/3. In terms of H0 and z rather than t0 and te , the current
proper distance is
dp (t0 ) =
2
c
[1 − (1 + z)−(1+3w)/2 ] .
H0 1 + 3w
(5.54)
The most distant object you can see (in theory) is one for which the light
emitted at t = 0 is just now reaching us at t = t0 . The proper distance (at
the time of observation) to such an object is called the horizon distance. 4
Here on Earth, the horizon is a circle centered on you, beyond which you
cannot see because of the Earth’s curvature. In the universe, the horizon is
a spherical surface centered on you, beyond which you cannot see because
light from more distant objects has not had time to reach you. In a universe
described by a Robertson-Walker metric, the current horizon distance is
dhor (t0 ) = c
Z
t0
0
dt
.
a(t)
(5.55)
In a spatially flat universe, the horizon distance has a finite value if w >
−1/3. In such a case, computing the value of dp (t0 ) in the limit te → 0 (or,
equivalently, z → ∞) yields
dhor (t0 ) = ct0
c
3(1 + w)
2
=
.
1 + 3w
H0 1 + 3w
(5.56)
In a flat universe dominated by matter (w = 0) or by radiation (w = 1/3), an
observer can see only a finite portion of the infinite volume of the universe.
The portion of the universe lying within the horizon for a particular observer
is referred to as the visible universe for that observer. The visible universe
4
More technically, this is what’s called the particle horizon distance; I’ll continue to
call it the horizon distance, for short.
94
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
consists of all points in space which have had sufficient time to send information, in the form of photons or other relativistic particles, to the observer. In
other words, the visible universe consists of all points which are causally connected to the observer; nothing which happens outside the visible universe
can have an effect on the observer.
In a flat universe with w ≤ −1/3, the horizon distance is infinite, and
all of space is causally connected to an observer. In such a universe with
w ≤ −1/3, you could see every point in space – assuming the universe was
transparent, of course. However, for extremely distant points, you would see
extremely redshifted versions of what they looked like extremely early in the
history of the universe.
5.4
Matter only
Let’s now look at specific examples of spatially flat universes, starting with
a universe containing only non-relativistic matter (w = 0).5 The age of such
a universe is
2
,
(5.57)
t0 =
3H0
and the horizon distance is
dhor (t0 ) = 3ct0 = 2c/H0 .
(5.58)
The scale factor, as a function of time, is
am (t) = (t/t0 )2/3 ,
(5.59)
illustrated as the dotted line in Figure 5.2. If you see a galaxy with redshift
z in a flat, matter-only universe, the proper distance to that galaxy, at the
time of observation, is
dp (t0 ) = c
Z
t0
te
"
2c
1
dt
= 3ct0 [1 − (te /t0 )1/3 ] =
1− √
2/3
(t/t0 )
H0
1+z
#
, (5.60)
illustrated as the dotted line in the upper panel of Figure 5.3. The proper
distance at the time the light was emitted was smaller by a factor 1/(1 + z):
"
2c
1
dp (te ) =
1− √
H0 (1 + z)
1+z
5
#
,
Such a universe is sometimes called an Einstein-de Sitter universe.
(5.61)
95
5.5. RADIATION ONLY
illustrated as the dotted line in the lower panel of Figure 5.3. In a flat, matteronly universe, dp (te ) has a maximum for galaxies with a redshift z = 5/4,
where dp (te ) = (8/27)c/H0 .
5.5
Radiation only
The case of a spatially flat universe containing only radiation is of particular
interest, since early in the history of our own universe, the radiation (w =
1/3) term dominated the right-hand side of the Friedmann equation (see
equation 5.27). Thus, at early times – long before the time of radiationmatter equality – the universe was well described by a spatially flat, radiationonly model. In an expanding, flat universe containing only radiation, the age
of the universe is
1
t0 =
,
(5.62)
2H0
and the horizon distance at t0 is
dhor (t0 ) = 2ct0 = c/H0 .
(5.63)
In the special case of a flat, radiation-only universe, the horizon distance is
exactly equal to the Hubble distance, which is not generally the case. The
scale factor of a flat, radiation-only universe is
a(t) = (t/t0 )1/2 ,
(5.64)
illustrated as the solid line in Figure 5.2. If, at a time t0 , you observe a
distant light source with redshift z in a flat, radiation-only universe, the
proper distance to the light source will be
dp (t0 ) = c
Z
t0
te
c
1
dt
1/2
=
2ct
[1
−
(t
/t
)
]
=
1
−
0
e
0
(t/t0 )1/2
H0
1+z
·
¸
,
(5.65)
illustrated as the solid line in the upper panel of Figure 5.3. The proper
distance at the time the light was emitted was
dp (te ) =
c
1
c
z
,
1−
=
H0 (1 + z)
1+z
H0 (1 + z)2
·
¸
(5.66)
illustrated as the solid line in the lower panel of Figure 5.3. In a flat,
radiation-dominated universe, dp (te ) has a maximum for light sources with a
redshift z = 1, where dp (te ) = (1/4)c/H0 .
96
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
The energy density in a flat, radiation-only universe is
εr (t) = ε0 (t/t0 )−2 =
3 EP
32 `3P
µ
t
tP
¶−2
≈ 0.094
EP
`3P
µ
t
tP
¶−2
.
(5.67)
Thus, in the early stages of our universe, when radiation was strongly dominant, the energy density, measured in units of the Planck density (EP /`3P ∼
3 × 10133 eV m−3 ), was comparable to one over the square of the cosmic time,
measured in units of the Planck time (tP ∼ 5×10−44 s). Using the blackbody
relation between energy density and temperature, given in equations (2.26)
and (2.27), we may assign a temperature to a universe dominated by blackbody radiation:
T (t) =
µ
45
32π 2
¶1/4
TP
µ
t
tP
¶−1/2
≈ 0.61TP
µ
t
tP
¶−1/2
.
(5.68)
Here TP is the Planck temperature, TP = 1.4 × 1032 K. The mean energy per
photon in a radiation-dominated universe is then
Emean (t) ≈ 2.70kT (t) ≈ 1.66EP
µ
t
tP
¶−1/2
,
(5.69)
and the number density of photons is (combining equations 5.67 and 5.69)
εr (t)
0.057 t
n(t) =
≈ 3
Emean (t)
`P
tP
µ
¶−3/2
.
(5.70)
Note that in a flat, radiation-only universe, as t → 0, εr → ∞ (equation 5.67).
Thus, at the instant t = 0, the energy density of our own universe (well
approximated as a flat, radiation-only model in its early stages) was infinite,
according to this analysis; this infinite energy density was provided by an
infinite number density of photons (equation 5.70), each of infinite energy
(equation 5.69). Should we take these infinities seriously? Not really, since
the assumptions of general relativity, on which the Friedmann equation is
based, break down at t ≈ tP . Thus, extrapolating the results of this chapter
to times earlier than the Planck time is not physically justified.
Why can’t general relativity be used at times earlier than the Planck time?
General relativity is a classical theory; that is, it does not take into account
the effects of quantum mechanics. In cosmological contexts, general relativity
assumes that the energy content of the universe is smooth down to arbitrarily
97
5.6. LAMBDA ONLY
small scales, instead of being parceled into individual quanta. As long as a
radiation-dominated universe has many, many quanta, or photons, within
a horizon distance, then the approximation of a smooth, continuous energy
density is justifiable, and we may safely use the results of general relativity.
However, if there are only a few photons within the visible universe, then
quantum mechanical effects must be taken into account, and the classical
results of general relativity no longer apply. In a flat, radiation-only universe,
the horizon distance grows linearly with time:
dhor (t) = 2ct = 2`P
µ
t
tP
¶
,
(5.71)
so the volume of the visible universe at time t is
Vhor (t) =
4π 3
d ≈ 34`3P
3 hor
µ
t
tP
¶3
.
(5.72)
Combining equations (5.72) and (5.70), we find that the number of photons
inside the horizon at time t is
N (t) = Vhor (t)n(t) ≈ 1.9
µ
t
tP
¶3/2
.
(5.73)
The quantization of the universe can no longer be ignored when N (t) ≈ 1,
equivalent to a time t ≈ 0.7tP .
In order to accurately described the universe at its very earliest stages,
prior to the Planck time, a theory of quantum gravity is needed. Unfortunately, a complete theory of quantum gravity does not yet exist. Consequently, in this book, I will not deal with times earlier than the Planck time,
t ∼ tP ∼ 5 × 10−44 s, when the number density of photons was n ∼ `−3
P ∼
2 × 10104 m−3 , and the mean photon energy was Emean ∼ EP ∼ 1 × 1028 eV.
5.6
Lambda only
As seen in Section 5.3, a spatially flat, single-component universe with w 6=
−1 has a power-law dependence of scale factor on time:
a ∝ t2/(3+3w) .
(5.74)
Now, for the sake of completeness, consider the case with w = −1; that
is, a universe in which the energy density is contributed by a cosmological
98
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
constant Λ.6 For such a flat, lambda-dominated universe, the Friedmann
equation takes the form
8πGεΛ 2
a ,
(5.75)
ȧ2 =
3c2
where εΛ is constant with time. This equation can be rewritten in the form
ȧ = H0 a ,
(5.76)
where
8πGεΛ 1/2
.
H0 =
3c2
The solution to equation (5.76) in an expanding universe is
¶
µ
a(t) = eH0 (t−t0 ) .
(5.77)
(5.78)
This scale factor is shown as the dot-dashed line in Figure 5.2. A spatially flat
universe with nothing but a cosmological constant is exponentially expanding; we’ve seen an exponentially expanding universe before, in Section 2.3,
under the label “Steady State universe”. In a Steady State universe, the density ε of the universe remains constant because of the continuous creation
of real particles. If the cosmological constant Λ is provided by the vacuum
energy, then the density ε of a lambda-dominated universe remains constant because of the continuous creation and annihilation of virtual particleantiparticle pairs.
A flat universe containing nothing but a cosmological constant is infinitely
old, and has an infinite horizon distance dhor . If, in a flat, lambda-only
universe, you see a light source with a redshift z, the proper distance to the
light source, at the time you observe it, is
dp (t0 ) = c
Z
t0
te
eH0 (t0 −t) dt =
c H0 (t0 −te )
c
[e
− 1] =
z,
H0
H0
(5.79)
shown as the dot-dashed line in the upper panel of Figure 5.3. The proper
distance at the time the light was emitted was
dp (te ) =
c z
,
H0 1 + z
shown as the dot-dashed line in the lower panel of Figure 5.3.
6
Such a universe is sometimes called a de Sitter universe.
(5.80)
5.6. LAMBDA ONLY
99
Note that an exponentially growing universe, such as the flat lambdadominated model, is the only universe for which dp (t0 ) is linearly proportional
to z for all values of z. In other universes, the relation dp (t0 ) ∝ z only holds
true in the limit z ¿ 1. Note also that in the limit z → ∞, dp (t0 ) → ∞
but dp (te ) → c/H0 . In a flat, lambda-dominated universe, highly redshifted
objects (z À 1) are at very large distances (dp (t0 ) À c/H0 ) at the time of
observation; the observer sees them as they were just before they reached a
proper distance c/H0 . Once a light source is more than a Hubble distance
from the observer, their recession velocity is greater than the speed of light,
and photons from the light source can no longer reach the observer.
The simple models that we’ve examined in this chapter – empty universes,
or flat universes with a single component – continue to expand forever if they
are expanding at t = t0 . Is it possible to have universes which stop expanding,
then start to collapse? Is it possible to have universes in which the scale
factor is not a simple power-law or exponential function of time? The short
answer to these questions is “yes”. To study universes with more complicated
behavior, however, it is necessary to put aside our simple toy universes, with
a single term on the right-hand side of the Friedmann equation, and look at
complicated toy universes, with multiple terms on the right-hand side of the
Friedmann equation.
Suggested reading
[Full references are given in the “Annotated Bibiography” on page 286.]
Liddle (1999), ch. 4: Flat universes, both matter-only and radiation-only
Linder (1997), ch. 2.4,2.5: Evolution of energy density; evolution of scale
factor in single-component universes
Problems
(5.1) The predicted number of neutrinos in the Cosmic Neutrino Background is nν = (3/11)nγ = 1.12 × 108 m−3 for each of the three
species of neutrino. About how many cosmic neutrinos are inside
your body right now? What must be the sum of the neutrino masses,
m(νe ) + m(νµ ) + m(ντ ), in order for the density of the Cosmic Neutrino
Background to be equal to the critical density, εc,0 ?
100
CHAPTER 5. SINGLE-COMPONENT UNIVERSES
(5.2) A light source in a flat, single-component universe has a redshift z
when observed at a time t0 . Show that the observed redshift changes
at a rate
dz
= H0 (1 + z) − H0 (1 + z)3(1+w)/2 .
(5.81)
dt0
For what values of w does the redshift decrease with time? For what
values of w does the redshift increase with time?
(5.3) Suppose you are in a flat, matter-only universe which has a Hubble
constant H0 = 70 km s−1 Mpc−1 . You observe a galaxy with z = 1.
How long will you have to keep observing the galaxy to see its redshift
change by one part in 106 ? [Hint: use the result from the previous
problem.]
(5.4) In a flat universe with H0 = 70 km s−1 Mpc−1 , you observe a galaxy
at a redshift z = 7. What is the current proper distance to the galaxy,
dp (t0 ), if the universe contains only radiation? What is dp (t0 ) if the
universe contains only matter? What is dp (t0 ) if the universe contains
only a cosmological constant? What was the proper distance at the
time the light was emitted, dp (te ), if the universe contains only radiation? What was dp (te ) if the universe contains only matter? What was
dp (te ) if the universe contains only a cosmological constant?
Chapter 6
Multiple-Component Universes
The Friedmann equation, in general, can be written in the form
H(t)2 =
8πG
κc2
ε(t)
−
,
3c2
R02 a(t)2
(6.1)
where H ≡ ȧ/a, and ε(t) is the energy density contributed by all the components of the universe, including the cosmological constant. Equation (4.31)
tells us the relation among κ, R0 , H0 , and Ω0 ,
H02
κ
(Ω0 − 1) ,
=
R02
c2
(6.2)
so we can rewrite the Friedmann equation without explicitly including the
curvature:
8πG
H02
H(t)2 =
ε(t)
−
(Ω0 − 1) .
(6.3)
3c2
a(t)2
Dividing by H02 , this becomes
ε(t) 1 − Ω0
H(t)2
=
+
,
2
H0
εc,0
a(t)2
(6.4)
where the critical density today is
εc,0 ≡
εm
3c2 H02
.
8πG
(6.5)
We know that our universe contains matter, for which the energy density
has the dependence εm = εm,0 /a3 , and radiation, for which the energy
101
102
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
density has the dependence εr = εr,0 /a4 . Current evidence seems to indicate
the presence of a cosmological constant, with energy density εΛ = εΛ,0 =
constant. It is certainly possible that the universe contains other components
as well. For instance, as the 21st century began, some cosmologists were
investigating the properties of “quintessence”, a component of the universe
whose equation-of-state parameter can lie in the range −1 < w < −1/3,
giving a universe with ä > 0. However, in the absence of strong evidence
for the existence of “quintessence”, I will only consider the contributions
of matter (w = 0), radiation (w = 1/3), and the cosmological constant Λ
(w = −1).
In our universe, we expect the Friedmann equation (6.4) to take the form
1 − Ω0
Ωr,0 Ωm,0
H2
,
= 4 + 3 + ΩΛ,0 +
2
H0
a
a
a2
(6.6)
where Ωr,0 = εr,0 /εc,0 , Ωm,0 = εm,0 /εc,0 , ΩΛ,0 = εΛ,0 /εc,0 , and Ω0 = Ωr,0 +
Ωm,0 + ΩΛ,0 . The Benchmark Model, introduced in the previous chapter as a
model consistent with all available data, has Ω0 = 1, and hence is spatially
flat. However, although a perfectly flat universe is consistent with the data,
it is not demanded by the data. Thus, prudence dictates that we should keep
in mind the possibility that the curvature term, (1−Ω0 )/a2 in equation (6.6),
might be nonzero.
Since H = ȧ/a, multiplying equation (6.6) by a2 , then taking the square
root, yields
H0−1 ȧ = [Ωr,0 /a2 + Ωm,0 /a + ΩΛ,0 a2 + (1 − Ω0 )]1/2 .
(6.7)
The cosmic time t as a function of scale factor a can then be found by
performing the integral
Z
a
0
[Ωr,0
/a2
da
= H0 t .
+ Ωm,0 /a + ΩΛ,0 a2 + (1 − Ω0 )]1/2
(6.8)
This is not a user-friendly integral: in the general case, it doesn’t have a
simple analytic solution. However, for given values of Ωr,0 , Ωm,0 , and ΩΛ,0 , it
can be integrated numerically.
In many circumstances, the integral in equation (6.8) has a simple analytic
approximation to its solution. For instance, as noted in the previous chapter,
in a universe with radiation, matter, curvature, and Λ, the radiation term
103
dominates the expansion during the early stages of expansion. In this limit,
equation (6.8) simplifies to
H0 t ≈
or
Z
a
0
ada
q
Ωr,0
q
1
≈ q
a2 ,
2 Ωr,0
a(t) ≈ (2 Ωr,0 H0 t)1/2 .
(6.9)
(6.10)
In the limit Ωr,0 = 1, this reduces to the solution already found for a flat,
radiation-only universe. If the universe continues to expand forever, then
in the limit a → ∞, the cosmological constant term will dominate the expansion. For some values of Ωr,0 , Ωm,0 , and ΩΛ,0 , there will be intermediate
epochs when the matter or the curvature dominates the expansion. For instance, in the Benchmark Model, where radiation-matter equality takes place
at a scale factor arm ≈ 2.8 × 10−4 and matter-lambda equality takes place at
amΛ ≈ 0.75, a matter-only universe is a fair approximation to reality when
arm ¿ a ¿ amΛ .
However, during some epochs of the universe’s expansion, two of the
components are of comparable density, and provide terms of roughly equal
size in the Friedmann equation. During these epochs, a single-component
model is a poor description of the universe, and a two-component model
must be utilized. For instance, for scale factors near arm ≈ 2.8 × 10−4 , the
Benchmark Model is well approximated by a flat universe containing only
radiation and matter. Such a universe is examined in Section 6.4. For scale
factors near amΛ ≈ 0.75, the Benchmark Model is well approximated by a
flat universe containing only matter and a cosmological constant. Such a
universe is examined in Section 6.2.
First, however, we will examine a universe which is of great historical interest to cosmology; a universe containing both matter and curvature (either
negative or positive). After Einstein dismissed the cosmological constant
as a blunder, and before astronomers had any clear idea what the value
of the density parameter Ω was, they considered the possibility that the
universe was negatively curved or positively curved, with the bulk of the
density being provided by non-relativistic matter. During the mid-twentieth
century, cosmologists concentrated much of their interest on the study of
curved, matter-dominated universes. In addition to being of historical interest, curved, matter-dominated universes provide useful physical insight into
the interplay among curvature, expansion, and density.
104
6.1
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
Matter + curvature
Consider a universe containing nothing but pressureless matter, with w = 0.
If such a universe is spatially flat, then it expands with time, as demonstrated
in Section 5.4, with a scale factor
a(t) = (t/t0 )2/3 .
(6.11)
Such a flat, matter-only universe expands outward forever. Such a fate is
sometimes known as the “Big Chill”, since the temperature of the universe
decreases monotonically with time as the universe expands. At this point, it
is nearly obligatory for a cosmology text to quote T. S. Eliot: “This is the
way the world ends / Not with a bang but a whimper.”1
In a curved universe containing nothing but matter, the ultimate fate
of the cosmos is intimately linked to the density parameter Ω0 . The Friedmann equation in a curved, matter-dominated universe (equation 6.6) can
be written in the form
Ω0 1 − Ω 0
H(t)2
,
= 3 +
2
H0
a
a2
(6.12)
since Ωm,0 = Ω0 in such a universe. Suppose you are in a universe which is
currently expanding (H0 > 0) and which contains nothing but non-relativistic
matter. If you ask the question, “Will the universe ever cease to expand?”
then equation (6.12) enables you to answer that question. For the universe
to cease expanding, there must be some moment at which H(t) = 0. Since
the first term on the right hand side of equation (6.12) is always positive,
H(t) = 0 requires the second term on the right hand side to be negative.
This means that a matter-dominated universe will cease to expand if Ω0 > 1,
and hence κ = +1. At the time of maximum expansion, H(t) = 0 and thus
0=
Ω0
1 − Ω0
+ 2
.
3
amax
amax
(6.13)
The scale factor at the time of maximum expansion will therefore be
amax =
1
Ω0
,
Ω0 − 1
(6.14)
Interestingly, this quote is from Eliot’s poem The Hollow Men, written, for the most
part, in 1924, the year when Friedmann published his second paper on the expansion of
the universe. However, this coincidence seems to be just that – a coincidence. Eliot did
not keep up to date on the technical literature of cosmology.
6.1. MATTER + CURVATURE
105
where Ω0 , remember, is the density parameter as measured at a scale factor
a = 1.
Note that in equation (6.12), the Hubble parameter enters only as H 2 .
Thus, the contraction phase, after the universe reaches maximum expansion,
is just the time reversal of the expansion phase.2 Eventually, the Ω0 > 1
universe will collapse down to a = 0 (an event sometimes called the “Big
Crunch”) after a finite time t = tcrunch . A matter-dominated universe with
Ω0 > 1 not only has finite spatial extent, but also has a finite duration in
time; just as it began in a hot, dense state, so it will end in a hot, dense
state. When such a universe is in its contracting stage, an observer will
see galaxies with a blueshift proportional to their distance. As the universe
approaches the Big Crunch, the cosmic microwave background will become a
cosmic infrared background, then a cosmic visible background, then a cosmic
ultraviolet background, then a cosmic x-ray background, then finally a cosmic
gamma-ray background.
A matter-dominated universe with Ω0 > 1 will expand to a maximum
scale factor amax , then collapse in a Big Crunch. What is the ultimate fate
of a matter-dominated universe with Ω0 < 1 and κ = −1? In the Friedmann
equation for such a universe (equation 6.12), both terms on the right hand
side are positive. Thus if such a universe is expanding at a time t = t0 ,
it will continue to expand forever. At early times, when the scale factor is
small (a ¿ Ω0 /[1 − Ω0 ]), the matter term of the Friedmann equation will
dominate, and the scale factor will grow at the rate a ∝ t2/3 . Ultimately,
however, the density of matter will be diluted far below the critical density,
and the universe will expand like the negatively curved empty universe, with
a ∝ t.
If a universe contains nothing but matter, its curvature, its density, and
its ultimate fate are closely linked, as shown in Table 6.1. At this point, the
obligatory quote is from Robert Frost: “Some say the world will end in fire /
Some say in ice.”3 In a matter-dominated universe, if the density is greater
2
The contraction is a perfect time reversal of the expansion only when the universe
is perfectly homogeneous and the expansion is perfectly adiabatic, or entropy-conserving.
In a real, lumpy universe, entropy is not conserved on small scales. Stars, for instance,
generate entropy as they emit photons. During the contraction phase of an Ω 0 > 1
universe, small-scale entropy-producing processes will NOT be reversed. Stars will not
absorb the photons which they previously emitted; people will not live backward from
grave to cradle.
3
This is from Frost’s poem Fire and Ice, first published in Harper’s Magazine in De-
106
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
Table 6.1: Curved, matter-dominated universes
density curvature
ultimate fate
Ω0 < 1
κ = −1
Big Chill (a ∝ t)
Ω0 = 1
κ=0
Big Chill (a ∝ t2/3 )
Ω0 > 1
κ = +1
Big Crunch
than the critical density, the universe will end in a fiery Big Crunch; if the
density is less than or equal to the critical density, the universe will end in
an icy Big Chill.
In a curved universe containing only matter, the scale factor a(t) can be
computed explicitly. The Friedmann equation can be written in the form
ȧ2
= Ω0 /a + (1 − Ω0 ) ,
H02
(6.15)
so the age t of the universe at a given scale factor a is given by the integral
H0 t =
Z
a
0
da
.
[Ω0 /a + (1 − Ω0 )]1/2
(6.16)
When Ω0 6= 1, the solution to this integral is most compactly written in a
parametric form. The solution when Ω0 > 1 is
1 Ω0
(1 − cos θ)
2 Ω0 − 1
(6.17)
1
Ω0
(θ − sin θ) ,
2H0 (Ω0 − 1)3/2
(6.18)
a(θ) =
and
t(θ) =
where the parameter θ runs from 0 to 2π. Given this parametric form, it is
easy to show that the time that elapses between the Big Bang at θ = 0 and
the Big Crunch at θ = 2π is
tcrunch =
Ω0
π
.
H0 (Ω0 − 1)3/2
(6.19)
A plot of a versus t in the case Ω0 = 1.1 is shown as the solid line in Figure 6.1.
The a ∝ t2/3 behavior of an Ω0 = 1 universe is shown as the dotted line. The
cember 1920. Unlike T. S. Eliot, Frost was keenly interested in astronomy, and frequently
wrote poems on astronomical themes.
107
6.1. MATTER + CURVATURE
40
Ω0=0.9
a
30
Ω0=1.0
20
Ω0=1.1
10
0
0
20
40
60
H0(t−t0)
80
100
2.5
2
a
1.5
1
.5
0
−.5
0
.5
1
H0(t−t0)
1.5
2
Figure 6.1: The scale factor as a function of time for universes containing
only matter. The dotted line is a(t) for a universe with Ω0 = 1 (flat); the
dashed line is a(t) for a universe with Ω0 = 0.9 (negatively curved); the solid
line is a(t) for a universe with Ω0 = 1.1 (positively curved). The bottom
panel is a blow-up of the small rectangle near the lower left corner of the
upper panel.
108
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
solution of equation (6.16) for the case Ω0 < 1 can be written in parametric
form as
1 Ω0
a(η) =
(cosh η − 1)
(6.20)
2 1 − Ω0
and
Ω0
1
(sinh η − η) ,
(6.21)
t(η) =
2 (Ω0 − 1)3/2
where the parameter η runs from 0 to infinity. A plot of a versus t in the
case Ω0 = 0.9 is shown as the dashed line in Figure 6.1. Note that although
the ultimate fates of an Ω0 = 0.9 universe is very different from that of an
Ω0 = 1.1 universe, as shown graphically in the upper panel of Figure 6.1, it
is very difficult, at t ∼ t0 , to tell a universe with Ω0 slightly less than one
from that with Ω0 slightly greater than one. As shown in the lower panel
of Figure (6.1), the scale factors of the Ω0 = 1.1 universe and the Ω0 = 0.9
universe start to diverge significantly only after a Hubble time or more.
Scientists sometimes joke that they are searching for a theory of the universe that is compact enough to fit on the front of a T-shirt. If the energy
content of the universe is contributed almost entirely by non-relativistic matter, then an appropriate T-shirt slogan would be:
DENSITY
IS
DESTINY!
If the density of matter is less than the critical value, then the destiny of
the universe is an ever-expanding Big Chill; if the density is greater than the
critical value, then the destiny is a recollapsing Big Crunch. Like all terse
summaries of complex concepts, the slogan “Density is Destiny!” requires
a qualifying footnote. In this case, the required footnote is “∗ if Λ = 0”. If
the universe has a cosmological constant (or more generally, any component
with w < −1/3), then the equation Density = Destiny = Curvature no longer
applies.
6.2
Matter + lambda
Consider a universe which is spatially flat, but which contains both matter
and a cosmological constant.4 If, at a given time t = t0 , the density parameter
4
Such a universe is of particular interest to us, since it appears to be a close approximation to our own universe at the present day.
109
6.2. MATTER + LAMBDA
in matter is Ωm,0 and the density parameter in a cosmological constant Λ is
ΩΛ,0 , the requirement that space be flat tells us that
ΩΛ,0 = 1 − Ωm,0 ,
(6.22)
and the Friedmann equation for the flat “matter plus lambda” universe reduces to
Ωm,0
H2
= 3 + (1 − Ωm,0 ) .
(6.23)
2
H0
a
The first term on the right hand side of equation (6.23) represents the contribution of matter, and is always positive. The second term represents the
contribution of a cosmological constant; it is positive if Ωm,0 < 1, implying
ΩΛ,0 > 0, and negative if Ωm,0 > 1, implying ΩΛ,0 < 0. Thus, a flat universe
with ΩΛ,0 > 0 will continue to expand forever if it is expanding at t = t0 ;
this is another example of a Big Chill universe. In a universe with ΩΛ,0 < 0,
however, the negative cosmological constant provides an attractive force, not
the repulsive force of a positive cosmological constant. A flat universe with
ΩΛ,0 < 0 will cease to expand at a maximum scale factor
amax =
Ã
Ωm,0
Ωm,0 − 1
!1/3
,
(6.24)
and will collapse back down to a = 0 at a cosmic time
tcrunch =
2π
1
q
.
3H0 Ωm,0 − 1
(6.25)
For a given value of H0 , the larger the value of Ωm,0 , the shorter the lifetime of
the universe. For a flat, ΩΛ,0 < 0 universe, the Friedmann equation (eq.6.23)
can be integrated to yield the analytic solution
h
i
2
H0 t = q
sin−1 (a/amax )3/2 .
3 Ωm,0 − 1
(6.26)
A plot of a versus t in the case Ωm,0 = 1.1, ΩΛ,0 = −0.1 is shown as the solid
line in Figure 6.2. The a ∝ t2/3 behavior of a Ωm,0 = 1, ΩΛ,0 = 0 universe
is shown, for comparison, as the dotted line. A flat universe with ΩΛ,0 < 0
ends in a Big Crunch, reminiscent of that for a positively curved, matteronly universe. However, with a negative cosmological constant providing an
110
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
40
Ωm,0=0.9
a
30
Ωm,0=1.0
20
10
0
Ωm,0=1.1
0
20
40
60
H0(t−t0)
80
100
Figure 6.2: The scale factor as a function of time for flat universes containing both matter and a cosmological constant. The dotted line is a(t) for
a universe with Ωm,0 = 1, ΩΛ,0 = 0. The solid line is a(t) for a universe
with Ωm,0 = 1.1, ΩΛ,0 = −0.1. The dashed line is a(t) for a universe with
Ωm,0 = 0.9, ΩΛ,0 = 0.1.
111
6.2. MATTER + LAMBDA
attractive force, the lifetime of a flat universe with ΩΛ,0 < 0 is exceptionally
short. For instance, we have seen that a positively curved universe with
Ωm,0 = 1.1 undergoes a Big Crunch after a lifetime tcrunch ≈ 110H0−1 (see
Figure 6.1). However, a flat universe with Ωm,0 = 1.1 and ΩΛ,0 = −0.1
has a lifetime tcrunch ≈ 7H0−1 . As soon as the universe becomes lambdadominated, the negative cosmological constant causes a rapid deceleration of
the universe’s expansion.
Although a negative cosmological constant is permitted by the laws of
physics, it appears that we live in a universe where the cosmological constant
is non-negative. In a flat universe with Ωm,0 < 1 and ΩΛ,0 > 0, the density
contributions of matter and the cosmological constant are equal at the scale
factor (eq. 5.22):
amΛ =
Ã
Ωm,0
ΩΛ,0
!1/3
=
Ã
Ωm,0
1 − Ωm,0
!1/3
.
(6.27)
For a flat, ΩΛ,0 > 0 universe, the Friedmann equation can be integrated to
yield the analytic solution
q
2
H0 t = q
ln (a/amΛ )3/2 + 1 + (a/amΛ )3 .
3 1 − Ωm,0
·
¸
(6.28)
A plot of a versus t in the case Ωm,0 = 0.9, ΩΛ,0 = 0.1 is shown as the dashed
line in Figure 6.2. At early times, when a ¿ amΛ , equation (6.28) reduces to
the relation
¶2/3
µ q
3
Ωm,0 H0 t
,
(6.29)
a(t) ≈
2
giving the a ∝ t2/3 dependence required for a flat, matter-dominated universe.
At late times, when a À amΛ , equation (6.28) reduces to
q
a(t) ≈ amΛ exp( 1 − Ωm,0 H0 t) ,
(6.30)
giving the a ∝ eKt dependence required for a flat, lambda-dominated universe. Suppose you are in a flat universe containing nothing but matter and
a cosmological constant; if you measure H0 and Ωm,0 , then equation (6.28)
tells you that the age of the universe is
2H0−1
q
t0 = q
ln 
3 1 − Ωm,0
1 − Ωm,0 + 1
q
Ωm,0

 .
(6.31)
112
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
If we approximate our own universe as having Ωm,0 = 0.3 and ΩΛ,0 = 0.7
(ignoring the contribution of radiation) we find that its current age is
t0 = 0.964H0−1 = 13.5 ± 1.3 Gyr ,
(6.32)
assuming H0 = 70 ± 7 km s−1 Mpc−1 . (We’ll see in section 6.5 that ignoring
the radiation content of the universe has an insignificant effect on our estimate of t0 .) The age at which matter and the cosmological constant had
equal energy density was
√
2H −1
tmΛ = q 0
ln[1 + 2] = 0.702H0−1 = 9.8 ± 1.0 Gyr .
3 1 − Ωm,0
(6.33)
Thus, if our universe is well fit by the Benchmark Model, with Ωm,0 = 0.3 and
ΩΛ,0 ≈ 0.7, then the cosmological constant has been the dominant component
of the universe for the last four billion years or so.
6.3
Matter + curvature + lambda
If a universe contains both matter and a cosmological constant, then the
formula “density = destiny = curvature” no longer holds. A flat universe
with Ωm,0 > 1 and ΩΛ,0 < 0, as shown in the previous section, is infinite in
spatial extent, but has a finite duration in time. By contrast, a flat universe
with Ωm,0 ≤ 1 and ΩΛ,0 ≥ 0 extends to infinity both in space and in time.
If a universe containing both matter and lambda is curved (κ 6= 0) rather
than flat, then a wide range of behaviors is possible for the function a(t).
For instance, in section 4.4, we encountered Einstein’s static model, in which
κ = +1 and εΛ = εm /2. A universe described by Einstein’s static model is
finite in spatial extent, but has infinite duration in time.
By choosing different values of Ωm,0 and ΩΛ,0 , without constraining the
universe to be flat, we can create model universes with scale factors a(t) which
exhibit very interesting behavior. Start by writing down the Friedmann
equation for a curved universe with both matter and a cosmological constant:
H2
Ωm,0 1 − Ωm,0 − ΩΛ,0
+ ΩΛ,0 .
= 3 +
2
H0
a
a2
(6.34)
If Ωm,0 > 0 and ΩΛ,0 > 0, then both the first and last term on the right hand
side of equation (6.34) are positive. However, if Ωm,0 + ΩΛ,0 > 1, so that the
6.3. MATTER + CURVATURE + LAMBDA
113
universe is positively curved, then the central term on the right hand side
is negative. As a result, for some choices of Ωm,0 and ΩΛ,0 , the value of H 2
will be positive for small values of a (where matter dominates) and for large
values of a (where Λ dominates), but will be negative for intermediate values
of a (where the curvature term dominates). Since negative values of H 2 are
unphysical, this means that these universes have a forbidden range of scale
factors. Suppose such a universe starts out with a À 1 and H < 0; that
is, it is contracting from a low-density, Λ-dominated state. As the universe
contracts, however, the negative curvature term in equation 6.34 becomes
dominant, causing the contraction to stop at a minimum scale factor a = amin ,
and then expand outward again in a “Big Bounce”. Thus, it is possible to
have a universe which expands outward at late times, but which never had an
initial Big Bang, with a = 0 at t = 0. Another possibility, if the values of Ω m,0
and ΩΛ,0 are chosen just right, is a “loitering” universe.5 Such a universe
starts in a matter-dominated state, expanding outward with a ∝ t2/3 . Then,
however, it enters a stage (called the loitering stage) in which a is very nearly
constant for a long period of time. During this time it is almost – but not
quite – Einstein’s static universe. After the loitering stage, the cosmological
constant takes over, and the universe starts to expand exponentially.
Figure 6.3 shows the general behavior of the scale factor a(t) as a function
of Ωm,0 and ΩΛ,0 . In the region labeled “Big Crunch”, the universe starts
with a = 0 at t = 0, reaches a maximum scale factor amax , then recollapses
to a = 0 at a finite time t = tcrunch . Note that Big Crunch universes can
be positively curved, negatively curved, or flat. In the region labeled “Big
Chill”, the universe starts with a = 0 at t = 0, then expands outward
forever, with a → ∞ as t → ∞. Like Big Crunch universes, Big Chill
universes can have any sign for their curvature. In the region labeled “Big
Bounce”, the universe starts in a contracting state, reaches a minimum scale
factor a = amin > 0 at some time tbounce , then expands outward forever, with
a → ∞ as t → ∞. Universes which fall just below the dividing line between
Big Bounce universes and Big Chill universes are loitering universes. The
closer such a universe lies to the Big Bounce – Big Chill dividing line in
Figure 6.3, the longer its loitering stage lasts.
To illustrate the different types of expansion and contraction possible,
Figure 6.4 shows a(t) for a set of four model universes. Each of these universes
has the same current density parameter for matter: Ωm,0 = 0.3, measured at
5
A loitering universe is sometimes referred to as a Lemaı̂tre universe.
114
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
3
Big
Bounce
ΩΛ,0
loi
te
rin
g
2
1
Big Chill
0
Big Crunch
κ=−1
−1
0
.5
1
1.5
Ωm,0
κ=+1
2
2.5
Figure 6.3: The curvature and type of expansion for universes containing
both matter and a cosmological constant. The dashed line indicates κ = 0;
models lying above this line have κ = +1, and those lying below have κ = −1.
Also shown are the regions where the universe has a “Big Chill” expansion
(a → ∞ as t → ∞), a “Big Crunch” recollapse (a → 0 as t → tcrunch ),
a loitering phase (a ∼ const for an extended period), or a “Big Bounce”
(a = amin > 0 at t = tbounce ).
115
6.3. MATTER + CURVATURE + LAMBDA
3
ΩΛ,0=0.7
2
ΩΛ,0=−0.3
a
ΩΛ,0=1.8
1
0
ΩΛ,0=1.7134
−4
−2
0
H0(t−t0)
2
4
Figure 6.4: The scale factor a as a function of t in four different universes,
each with Ωm,0 = 0.3. The dashed line shows a “Big Crunch” universe
(ΩΛ,0 = −0.3, κ = −1). The dotted line shows a “Big Chill” universe
(ΩΛ,0 = 0.7, κ = 0). The dot-dash line shows a loitering universe (ΩΛ,0 =
1.7134, κ = +1). The solid line shows a “Big Bounce” universe (ΩΛ,0 = 1.8,
κ = +1).
116
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
t = t0 and a = 1. These universes cannot be distinguished from each other by
measuring their current matter density and Hubble constant. Nevertheless,
thanks to their different values for the cosmological constant, they have very
different pasts and very different futures. The dashed line in Figure 6.4
shows the scale factor for a universe with ΩΛ,0 = −0.3; this universe has
negative curvature, and is destined to end in a Big Crunch. The dotted
line shows a(t) for a universe with ΩΛ,0 = 0.7; this universe is spatially
flat, and is destined to end in an exponentially expanding Big Chill . The
dot-dash line shows the scale factor for a universe with ΩΛ,0 = 1.7134; this
is a positively curved loitering universe, which spends a long time with a
scale factor a ≈ aloiter ≈ 0.44. Finally, the solid line shows a universe with
ΩΛ,0 = 1.8. This universe lies above the Big Chill – Big Bounce dividing line
in Figure 6.3; it is a positively curved universe which “bounced” at a scale
factor a = abounce ≈ 0.56.
There is strong observational evidence that we do not live in a loitering
or Big Bounce universe. If we lived in a loitering universe, then as we looked
out into space, we would see nearly the same redshift zloiter = 1/aloiter − 1 for
galaxies with a very large range of distances. For instance, with aloiter ≈ 0.44
(the appropriate loitering scale factor for a universe with Ωm,0 = 0.3), this
would lead to a large excess of galaxies with zloiter ≈ 1.3. No such excess of
galaxies is seen at any redshift in our universe. If we lived in a Big Bounce
universe, then the largest redshift we would see for any galaxy would be
zmax = 1/abounce − 1. As we looked further into space, we would see redshifts
increase to zmax , then see the redshifts decrease until they actually became
blueshifts. In our universe, we do not see such distant blueshifted galaxies.
Our own universe seems to be a Big Chill universe, fated to eternal expansion.
6.4
Radiation + matter
In our universe, radiation-matter equality took place at a scale factor arm ≡
Ωr,0 /Ωm,0 ≈ 2.8 × 10−4 . At scale factors a ¿ arm , the universe is well
described by a flat, radiation-only model, as described in Section 5.5. At scale
factors a ∼ arm , the universe is better described by a flat model containing
both radiation and matter. The Friedmann equation around the time of
radiation-matter equality can be written in the approximate form
Ωr,0 Ωm,0
H2
= 4 + 3 .
2
H0
a
a
(6.35)
117
6.4. RADIATION + MATTER
This can be rearranged in the form
H0 dt =
ada
1/2
Ωr,0
[1 + a/arm ]−1/2 .
(6.36)
Integration yields a fairly simple relation for t as a function of a during the
epoch when only radiation and matter are significant:
4a2rm
H0 t = q
3 Ωr,0
"
a
1− 1−
2arm
µ
¶µ
a
1+
arm
¶1/2 #
.
(6.37)
In the limit a ¿ arm , this gives the appropriate result for the radiationdominated phase of evolution (compare equation 6.10),
³ q
a ≈ 2 Ωr,0 H0 t
´1/2
[a ¿ arm ] .
(6.38)
In the limit a À arm (but before curvature or Λ contributes significantly to
the Friedmann equation), the approximate result for a(t) becomes
a≈
µ q
3
2
Ωm,0 H0 t
¶2/3
[a À arm ] .
(6.39)
The time of radiation-matter equality, trm , can be found by setting a = arm
in equation (6.37):
3/2
trm
Ω
1 a2
4
= (1 − √ ) q rm H0−1 ≈ 0.391 2r,0 H0−1 .
3
Ωm,0
2 Ωr,0
(6.40)
For the Benchmark Model, with Ωr,0 = 8.4 × 10−5 , Ωm,0 = 0.3, and H0−1 =
14 Gyr, the time of radiation-matter equality was
trm = 3.34 × 10−6 H0−1 = 47,000 yr .
(6.41)
The epoch when the universe was radiation-dominated was only about 47
millennia long. This is sufficiently brief that it justifies our ignoring the
effects of radiation when computing the age of the universe. The age t0 =
0.964H0−1 = 13.5 Gyr that we computed in Section 6.2 (ignoring radiation)
would only be altered by a few parts per million if we included the effects of
radiation. This minor correction is dwarfed by the 10% uncertainty in the
value of H0−1 .
118
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
Table 6.2: Properties of the Benchmark Model
photons:
neutrinos:
total radiation:
baryonic matter:
nonbaryonic dark matter:
total matter:
cosmological constant:
radiation-matter equality:
matter-lambda equality:
Now:
6.5
List of Ingredients
Ωγ,0 = 5.0 × 10−5
Ων,0 = 3.4 × 10−5
Ωr,0 = 8.4 × 10−5
Ωbary,0 = 0.04
Ωdm,0 = 0.26
Ωm,0 = 0.30
ΩΛ,0 ≈ 0.70
Important Epochs
arm = 2.8 × 10−4
amΛ = 0.75
a0 = 1
trm = 4.7 × 104 yr
tmΛ = 9.8 Gyr
t0 = 13.5 Gyr
Benchmark Model
The Benchmark Model, which I have adopted as the best fit to the currently
available observational data, is spatially flat, and contains radiation, matter,
and a cosmological constant. Some of its properties are listed, for ready
reference, in Table 6.2. The Hubble constant of the Benchmark Model is
assumed to be H0 = 70 km s−1 Mpc−1 . The radiation in the Benchmark
Model consists of photons and neutrinos. The photons are assumed to be
provided solely by a Cosmic Microwave Background with current temperature
T0 = 2.725 K and density parameter Ωγ,0 = 5.0 × 10−5 . The energy density of
the cosmic neutrino background is theoretically calculated to be 68% of that
of the Cosmic Microwave Background, as long as neutrinos are relativistic.
If a neutrino has a non-zero mass mν , equation (5.18) tells us that it defects
from the “radiation” column to the “matter” column when the scale factor
is a ∼ 5 × 10−4 eV/(mν c2 ). The matter content of the Benchmark Model
consists partly of baryonic matter (that is, matter composed of protons and
neutrons, with associated electrons), and partly of nonbaryonic dark matter.
As we’ll see in future chapters, the evidence indicates that most of the matter
in the universe is nonbaryonic dark matter. The baryonic material that we
are familiar with from our everyday existence has a density parameter of
Ωbary,0 ≈ 0.04 today. The density parameter of the nonbaryonic dark matter
119
6.5. BENCHMARK MODEL
2
a∝e
Kt
log(a)
0
2/3
a∝t
−2
−4
−6
−10
tmΛ
1/2
a∝t
−8
t0
trm
−6
−4
log(H0t)
−2
0
Figure 6.5: The scale factor a as a function of time t (measured in units
of the Hubble time), computed for the Benchmark Model. The dotted lines
indicate the time of radiation-matter equality, arm = 2.8 × 10−4 , the time of
matter-lambda equality, amΛ = 0.75, and the present moment, a0 = 1.
is roughly six times greater: Ωdm,0 ≈ 0.26. The bulk of the energy density in
the Benchmark Model, however, is not provided by radiation or matter, but
by a cosmological constant, with ΩΛ,0 = 1 − Ωm,0 − Ωr,0 ≈ 0.70.
The Benchmark Model was first radiation-dominated, then matter-dominated,
and is now entering into its lambda-dominated phase. As we’ve seen, radiation gave way to matter at a scale factor arm = Ωr,0 /Ωm,0 = 2.8 × 10−4 ,
corresponding to a time trm = 4.7 × 104 yr. Matter, in turn, gave way to
the cosmological constant at amΛ = (Ωm,0 /ΩΛ,0 )1/3 = 0.75, corresponding to
tmΛ = 9.8 Gyr. The current age of the universe, in the Benchmark Model, is
t0 = 13.5 Gyr.
With Ωr,0 , Ωm,0 , and ΩΛ,0 known, the scale factor a(t) can be computed
numerically using the Friedmann equation, in the form of equation (6.6).
Figure 6.5 shows the scale factor, thus computed, for the Benchmark Model.
Note that the transition from the a ∝ t1/2 radiation-dominated phase to
the a ∝ t2/3 matter-dominated phase is not an abrupt one; neither is the
later transition from the matter-dominated phase to the exponentially growing lambda-dominated phase. One curious feature of the Benchmark Model
120
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
which Figure 6.5 illustrates vividly is that we are living very close to the time
of matter-lambda equality.
Once a(t) is known, other properties of the Benchmark Model can readily
be computed. For instance, the upper panel of Figure 6.6 shows the current
proper distance to a galaxy with redshift z. The heavy solid line is the result
for the Benchmark Model; for purposes of comparison, the result for a flat
lambda-only universe is shown as a dot-dash line and the result for a flat
matter-only universe is shown as the dotted line. In the limit z → ∞, the
proper distance dp (t0 ) approaches a limiting value dp → 3.24c/H0 , in the case
of the Benchmark Model. Thus, the Benchmark Model has a finite horizon
distance,
dhor (t0 ) = 3.24c/H0 = 3.12ct0 = 14,000 Mpc .
(6.42)
If the Benchmark Model is a good description of our own universe, then we
can’t see objects more than 14 gigaparsecs away because light from them has
not yet had time to reach us. The lower panel of Figure 6.6 shows dp (te ),
the distance to a galaxy with observed redshift z at the time the observed
photons were emitted. For the Benchmark Model, dp (te ) has a maximum for
galaxies with redshift z = 1.6, where dp (te ) = 0.41c/H0 .
When astronomers observe a distant galaxy, they ask the related, but not
identical, questions, “How far away is that galaxy?” and “How long has the
light from that galaxy been traveling?” In the Benchmark Model, or any
other model, we can answer the question “How far away is that galaxy?” by
computing the proper distance dp (t0 ). We can answer the question “How long
has the light from that galaxy been traveling?” by computing the lookback
time. If light emitted at time te is observed at time t0 , the lookback time is
simply t0 −te . In the limits of very small redshifts, t0 −te ≈ z/H0 . However, as
shown in Figure 6.7, at larger redshifts, the relation between lookback time
and redshift becomes nonlinear. The exact dependence of lookback time
on redshift depends strongly on the cosmological model used. For example,
consider a galaxy with redshift z = 2. In the Benchmark Model, the lookback
time to that galaxy is 10.5 Gyr; we are seeing a redshifted image of that galaxy
as it was 10.5 billion years ago. In a flat, lambda-only universe, however, the
lookback time to a z = 2 galaxy is 15.4 Gyr, assuming H0−1 = 14 Gyr. In
a flat, matter-dominated universe, the lookback time to a z = 2 galaxy is a
mere 7.5 Gyr, with the same assumed Hubble constant. Knowing Ωm,0 and
ΩΛ,0 thus becomes important to studies of galaxy evolution. How long does
it take galaxies at z ≈ 2 to evolve into galaxies similar to those at z ≈ 0? Is
121
6.5. BENCHMARK MODEL
(H0/c) dp(t0)
100
Observation
Λ−only
10
Benchmark
1
matter−only
.1
.01
.01
(H0/c) dp(te)
100
.1
1
z
10
100 1000
10
100 1000
Emission
10
1
.1
.01
.01
.1
1
z
Figure 6.6: The proper distance to a light source with observed redshift
z. The upper panel shows the distance at the time of observation; the lower
panel shows the distance at the time of emission. The bold solid line indicates
the Benchmark Model, the dot-dash line a flat, lambda-only universe, and
the dotted line a flat, matter-only universe.
122
CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
Λ−only
15
t0−te (Gyr)
Benchmark
10
matter−only
5
0
0
2
z
4
6
Figure 6.7: The lookback time, t0 − te , for galaxies with observed redshift z.
The Hubble time is assumed to be H0−1 = 14 Gyr. The heavy solid line shows
the result for the Benchmark Model, the dot-dash line for a flat, lambda-only
universe, and the dotted line for a flat, matter-only universe.
it 15 billion years, or only half that time, or something in between? In future
years, as the Benchmark Model becomes better constrained, our ability to
translate observed redshifts into deduced times will become more accurate.
The most distant galaxies that have been observed (at the beginning of the
21st century) are at a redshift z ≈ 6. Consider such a high-redshift galaxy.
Using the Benchmark Model, we find that the current proper distance to a
galaxy with z = 6 is dp (t0 ) = 1.92c/H0 = 8300 Mpc, about 60% of the current
horizon distance. The proper distance at the time the light was emitted was
smaller by a multiplicative factor 1/(1 + z) = 1/7. This means that the
galaxy was only at a distance dp (te ) = 0.27c/H0 = 1200 Mpc at the time
the light was emitted. The light was emitted when the age of the universe
was te = 0.066H0−1 = 0.9 Gyr, or less than 7% of the universe’s current age,
t0 = 13.5 Gyr. The lookback time to a z = 6 galaxy in the Benchmark Model
is thus t0 − te = 12.6 Gyr. Astronomers are fond of saying, “A telescope
is a time machine.” As you look further and further out into the universe,
to objects with larger and larger values of dp (t0 ), you are looking back to
objects with smaller and smaller values of te . When you observe a galaxy
123
6.5. BENCHMARK MODEL
with a redshift z = 6, according to the Benchmark Model, you are glimpsing
the universe as it was as a youngster, less than a billion years old.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Harrison (2000), ch. 18: A classification of possible universes, by kinematic and dynamic criteria.
Kolb & Turner (1990), ch. 3.2: The scale factor a(t) and current age
t0 for various two-component universes; contains useful formulae and
graphs.
Linder (1997), ch. 4.5: The fate of the universe, for different values of the
equation-of-state parameter w.
Problems
(6.1) In a positively curved universe containing only matter (Ω0 > 1, κ =
+1), show that the present age of the universe is given by the formula
Ω0
2 − Ω0
1
H0 t 0 =
cos−1
.
−
3/2
2(Ω0 − 1)
Ω0
Ω0 − 1
µ
¶
(6.43)
Assuming H0 = 70 km s−1 Mpc−1 , plot t0 as a function of Ω0 in the
range 1 ≤ Ω0 ≤ 3.
(6.2) In a negatively curved universe containing only matter (Ω0 < 1, κ =
−1), show that the present age of the universe is given by the formula
H0 t 0 =
1
2 − Ω0
Ω0
cosh−1
−
3/2
1 − Ω0 2(1 − Ω0 )
Ω0
µ
¶
.
(6.44)
Assuming H0 = 70 km s−1 Mpc−1 , plot t0 as a function of Ω0 in the
range 0 ≤ Ω0 ≤ 1. The current best estimate for the ages of stars in
globular clusters yields an age of t = 13 Gyr for the oldest globular
clusters. In a matter-only universe, what is the maximum permissible
value of Ω0 , given the constraints H0 = 70 km s−1 Mpc−1 and t0 >
13 Gyr?
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CHAPTER 6. MULTIPLE-COMPONENT UNIVERSES
(6.3) One of the more recent speculations in cosmology is that the universe may contain a quantum field, called “quintessence”, which has
a positive energy density and a negative value of the equation-of-state
parameter w. Assume, for the purposes of this problem, that the universe is spatially flat, and contains nothing but matter (w = 0), and
quintessence with w = −1/2. The current density parameter of matter is Ωm,0 ≤ 1, and the current density parameter of quintessence is
ΩQ,0 = 1 − Ωm,0 . At what scale factor amQ will the energy density of
quintessence and matter be equal? Solve the Friedmann equation to
find a(t) for this universe. What is a(t) in the limit a ¿ amQ ? What
is a(t) in the limit a À amQ ? What is the current age of this universe,
expressed in terms of H0 and Ωm,0 ?
(6.4) Suppose you wanted to “pull an Einstein”, and create a static universe
(ȧ = 0, ä = 0) in which the gravitational attraction of matter is exactly
balanced by the gravitational repulsion of quintessence with equationof-state parameter wQ . Within what range must wQ fall for the effects
of quintessence to be repulsive? Is it possible to create a static universe
for an arbitrary value of wQ within this range? If so, will the curvature
of the universe be negative or positive? What energy density εQ will
be required, for a given value of wQ , to balance an energy density εm
in matter?
(6.5) Consider a positively curved universe containing only matter (the “Big
Crunch” model discussed in Section 6.1). At some time t0 > tCrunch /2,
during the contraction phase of this universe, an astronomer named
Elbbuh Niwde discovers that nearby galaxies have blueshifts (−1 ≤
z < 0) proportional to their distance. He then measures H0 and Ω0 ,
finding H0 < 0 and Ω0 > 1. Given H0 and Ω0 , how long a time will
elapse between Dr. Niwde’s observations at t = t0 and the final Big
Crunch at t = tcrunch ? What is the minimum blueshift that Dr. Niwde
is able to observe? What is the lookback time to an object with this
blueshift?
(6.6) Consider an expanding, positively curved universe containing only a
cosmological constant (Ω0 = ΩΛ,0 > 1). Show that such a universe
underwent a “Big Bounce” at a scale factor
abounce
Ω0 − 1
=
Ω0
µ
¶1/2
,
(6.45)
125
6.5. BENCHMARK MODEL
and that the scale factor as a function of time is
q
a(t) = abounce cosh[ Ω0 H0 (t − tbounce )] ,
(6.46)
where tbounce is the time at which the Big Bounce occurred. What is
the time t0 − tbounce which has elapsed since the Big Bounce, expressed
as a function of H0 and Ω0 ?
(6.7) A universe is spatially flat, and contains both matter and a cosmological constant. For what value of Ωm,0 is t0 exactly equal to H0−1 ?
(6.8) In the Benchmark Model, what is the total mass of all the matter
within our horizon? What is the total energy of all the photons within
our horizon? How many baryons are within the horizon?
Chapter 7
Measuring Cosmological
Parameters
Cosmologists would like to know the scale factor a(t) for the universe. For
a model universe whose contents are known with precision, the scale factor
can be computed from the Friedmann equation. Finding a(t) for the real
universe, however, is much more difficult. The scale factor is not directly observable; it can only be deduced indirectly from the imperfect and incomplete
observations that we make of the universe around us.
In the previous three chapters, I’ve pointed out that if we knew the energy
density ε for each component of the universe, we could use the Friedmann
equation to find the scale factor a(t). The argument works in the other
direction, as well; if we could determine a(t) from observations, we could
use that knowledge to find ε for each component. Let’s see, then, what
constraints we can put on the scale factor by making observations of distant
astronomical objects.
7.1
“A search for two numbers”
Since determining the exact functional form of a(t) is difficult, it is useful,
instead, to do a Taylor series expansion for a(t) around the present moment.
The complete Taylor series is
da ¯¯
1 d2 a ¯¯
a(t) = a(t0 ) +
¯
(t − t0 ) +
¯
(t − t0 )2 + . . .
dt ¯t=t0
2 dt2 ¯t=t0
¯
¯
126
(7.1)
127
7.1. “A SEARCH FOR TWO NUMBERS”
To exactly reproduce an arbitrary function a(t) for all values of t, an infinite number of terms is required in the expansion. However, the usefulness
of a Taylor series expansion resides in the fact that if a doesn’t fluctuate
wildly with t, using only the first few terms of the expansion gives a good
approximation in the immediate vicinity of t0 . The scale factor a(t) is a good
candidate for a Taylor expansion. The different model universes examined in
the previous two chapters all had smoothly varying scale factors, and there’s
no evidence that the real universe has a wildly oscillating scale factor.
Keeping the first three terms of the Taylor expansion, the scale factor in
the recent past and the near future can be approximated as
1 d2 a ¯¯
da ¯¯
(t − t0 )2 .
(t − t0 ) +
¯
¯
a(t) ≈ a(t0 ) +
dt ¯t=t0
2 dt2 ¯t=t0
¯
¯
(7.2)
Dividing by the current scale factor, a(t0 ),
a(t)
ȧ ¯
1 ä ¯¯
≈ 1 + ¯¯
(t − t0 ) +
(t − t0 )2 .
a(t0 )
a t=t0
2 a ¯t=t0
¯
¯
(7.3)
Using the normalization a(t0 ) = 1, this expansion for the scale factor is
customarily written in the form
1
a(t) ≈ 1 + H0 (t − t0 ) − q0 H02 (t − t0 )2 .
2
(7.4)
In equation (7.4), the parameter H0 is our old acquaintance the Hubble
constant,
¯
ȧ ¯¯
,
(7.5)
H0 ≡ ¯
a t=t0
and the parameter q0 is a dimensionless number called the deceleration parameter, defined as
äa
q0 ≡ − 2
ȧ
µ
¶
t=t0
ä
=−
aH 2
µ
¶
.
(7.6)
t=t0
Note the choice of sign in defining q0 . A positive value of q0 corresponds to
ä < 0, meaning that the universe’s expansion is decelerating (that is, the
relative velocity of any two points is decreasing). A negative value of q0
corresponds to ä > 0, meaning that the relative velocity of any two points is
increasing with time. The choice of sign for q0 , and the fact that it’s named
128
CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
the deceleration parameter, is due to the fact that it was first defined during
the mid-1950’s, when the limited information available favored a matterdominated universe with ä < 0. If the universe contains a sufficiently large
cosmological constant, however, the deceleration parameter q0 can have either
sign.
The Taylor expansion of equation (7.4) is physics-free. It is simply a
mathematical description of how the universe expands at times t ∼ t0 , and
says nothing at all about what forces act to accelerate the expansion (to
take a Newtonian viewpoint of the physics involved). The parameters H0
and q0 are thus purely descriptive of the kinematics, and are free of the theoretical “baggage” underlying the Friedmann equation and the acceleration
equation.1 In a famous 1970 review article, the observational cosmologist
Allan Sandage described all of cosmology as “A Search for Two Numbers”.
Those two numbers were H0 and q0 . Although the scope of cosmology has
widened considerably since Sandage wrote his article, cosmologists are still
assiduously searching for H0 and q0 .
Although H0 and q0 are themselves free of the theoretical assumptions
underlying the Friedmann and acceleration equations, you can use the acceleration equation to predict what q0 will be in a given model universe. If your
model universe contains several components, each with a different value of
the equation-of-state parameter w, the acceleration equation can be written
4πG X
ä
εw (1 + 3w) .
=− 2
a
3c w
(7.7)
·
¸
ä
1 8πG X
−
=
εw (1 + 3w) .
aH 2
2 3c2 H 2 w
(7.8)
Divide each side of the acceleration equation by the square of the Hubble
parameter H(t) and change sign:
However, the quantity in square brackets in equation (7.8) is just the inverse of the critical energy density εc . Thus, we can rewrite the acceleration
equation in the form
−
1
1X
ä
=
Ωw (1 + 3w) .
aH 2
2 w
(7.9)
Remember, the Friedmann equation assumes that the expansion of the universe is
controlled by gravity, and that gravity is accurately described by Einstein’s theory of
general relativity; although these are reasonable assumptions, they are not 100% ironclad.
7.1. “A SEARCH FOR TWO NUMBERS”
129
Evaluating equation (7.9) at the present moment, t = t0 , tells us the relation
between the deceleration parameter q0 and the density parameters of the
different components of the universe:
q0 =
1X
Ωw,0 (1 + 3w) .
2 w
(7.10)
For a universe containing radiation, matter, and a cosmological constant,
1
q0 = Ωr,0 + Ωm,0 − ΩΛ,0 .
2
(7.11)
Such a universe will currently be accelerating outward (q0 < 0) if ΩΛ,0 >
Ωr,0 + Ωm,0 /2. The Benchmark Model, for instance, has q0 ≈ −0.55.
In principle, determining H0 should be easy. For small redshifts, the relation between a galaxy’s distance d and its redshift z is linear (equation 2.5):
cz = H0 d .
(7.12)
Thus, if you measure the distance d and redshift z for a large sample of galaxies, and fit a straight line to a plot of cz versus d, the slope of the plot gives you
the value of H0 .2 Measuring the redshift of a galaxy is relatively simple; automated galaxy surveys can find hundreds of galaxy redshifts in a single night.
The difficulty is in measuring the distance of a galaxy. Remember, Edwin
Hubble was off by a factor of 7 when he estimated H0 ≈ 500 km s−1 Mpc−1
(see Figure 2.4). This is because he underestimated the distances to galaxies
in his sample by a factor of 7.
The distance to a galaxy is not only difficult to measure but also, in an
expanding universe, somewhat difficult to define. In Section 3.3, the proper
distance dp (t) between two points was defined as the length of the spatial
geodesic between the points when the scale factor is fixed at the value a(t).
The proper distance is perhaps the most straightforward definition of the
spatial distance between two points in an expanding universe. Moreover,
there is a helpful relation between scale factor and proper distance. If we
observe, at time t0 , light that was emitted by a distant galaxy at time te , the
2
The peculiar velocities of galaxies cause a significant amount of scatter in the plot,
but by using a large number of galaxies, you can beat down the statistical errors. If you
use galaxies at d < 100 Mpc, you must also make allowances for the local inhomogeneity
and anisotropy.
130
CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
current proper distance to that galaxy is (equation 5.35):
dp (t0 ) = c
Z
t0
te
dt
.
a(t)
(7.13)
For the model universes examined in Chapters 5 and 6, we knew the exact
functional form of a(t), and hence could exactly compute dp (t0 ) for a galaxy
of any redshift. If we have only partial knowledge of the scale factor, in the
form of the Taylor expansion of equation 7.4, we may use the expansion
1
≈ 1 − H0 (t − t0 ) + (1 + q0 /2)H02 (t − t0 )2
a(t)
(7.14)
in equation 7.13. Including the two lowest-order terms in the lookback time,
t0 − te , we find that the proper distance to the galaxy is
cH0
(t0 − te )2 .
(7.15)
2
The first term in the above equation, c(t0 − te ), is what the proper distance
would be in a static universe – the lookback time times the speed of light.
The second term is a correction due to the expansion of the universe during
the time the light was traveling.
Equation 7.15 would be extremely useful if the light from distant galaxies
carried a stamp telling us the lookback time, t0 − te . They don’t; instead,
they carry a stamp telling us the scale factor a(te ) at the time the light was
emitted. The observed redshift z of a galaxy, remember, is
dp (t0 ) ≈ c(t0 − te ) +
z=
1
−1 .
a(te )
(7.16)
Using equation 7.14, we may write an approximate relation between redshift
and lookback time:
z ≈ H0 (t0 − te ) + (1 + q0 /2)H02 (t0 − te )2 .
(7.17)
Inverting equation 7.17 to give the lookback time as a function of redshift,
we find
i
h
(7.18)
t0 − te ≈ H0−1 z − (1 + q0 /2)z 2 .
Substituting equation 7.18 into equation 7.15 gives us an approximate relation for the current proper distance to a galaxy with redshift z:
·
¸
i
c h
cH0 z 2
c
1 + q0
2
dp (t0 ) ≈
z − (1 + q0 /2)z +
=
z 1−
z .
H0
2 H02
H0
2
(7.19)
7.2. LUMINOSITY DISTANCE
131
The linear Hubble relation dp ∝ z thus only holds true in the limit z ¿
2/(1 + q0 ). If q0 > −1, then the proper distance to a galaxy of moderate
redshift (z ∼ 0.1, say) is less than would be predicted from the linear Hubble
relation.
7.2
Luminosity distance
Unfortunately, the current proper distance to a galaxy, dp (t0 ), is not a measurable property. If you tried to measure the distance to a galaxy with a tape
measure, for instance, the distance would be continuously increasing as you
extended the tape. To measure the proper distance at time t0 , you would
need a tape measure which could be extended with infinite speed; alternatively, you would need to stop the expansion of the universe at its current
scale factor while you measured the distance at your leisure. Neither of these
alternatives is physically possible.
Since cosmology is ultimately based on observations, if we want to find
the distance to a galaxy, we need some way of computing a distance from
that galaxy’s observed properties. In devising ways of computing the distance to galaxies, astronomers have found it useful to adopt and adapt the
techniques used to measure shorter distances. Let’s examine, then, the techniques used to measure relatively short distances. Within the Solar System,
astronomers measure the distance to the Moon and planets by reflecting
radar signals from them. If δt is the time taken for a photon to complete
the round-trip, then the distance to the reflecting body is d = (c δt)/2.3 The
accuracy with which distances have been determined with this technique is
impressive; the length of the astronomical unit, for instance, is now known
to be 1 AU = 149, 597, 870.61 km. The radar technique is useful only within
the Solar System. Beyond ∼ 10 AU, the reflected radio waves are too faint
to detect.
A favorite method for determining distances to other stars within our
Galaxy is the method of trigonometric parallax. When a star is observed
from two points separated by a distance b, the star’s apparent position will
shift by an angle θ. If the baseline of observation is perpendicular to the line
3
Since the relative speeds of objects within the Solar System are much smaller than c,
the corrections due to relative motion during the time δt are minuscule.
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
of sight to the star, the parallax distance will be
Ã
b
dπ = 1 pc
1 AU
!Ã
θ
1 arcsec
!−1
.
(7.20)
Measuring the distances to stars using the Earth’s orbit (b = 2 AU) as a
baseline is a standard technique. Since the size of the Earth’s orbit is known
with great accuracy from radar measurements, the accuracy with which the
parallax distance can be determined is limited by the accuracy with which θ
can be measured. The Hipparcos satellite, launched by the European Space
Agency in 1989, found the parallax distance for ∼ 105 stars, with an accuracy
of ∼ 1 milliarcsecond. However, to measure θ for a galaxy ∼ 100 Mpc away,
an accuracy of < 10 nanoarcseconds would be required, using the Earth’s
orbit as a baseline. The trigonometric parallaxes of galaxies at cosmological
distances are too small to be measured with current technology.
Let’s focus on the properties that we can measure for objects at cosmological distances. We can measure the flux of light, f , from the object, in
units of watts per square meter. The complete flux, integrated over all wavelengths of light, is called the bolometric flux. (A bolometer is an extremely
sensitive thermometer capable of detecting electromagnetic radiation over a
wide range of wavelengths; it was invented in 1881 by the astronomer Samuel
Langley, who used it to measure solar radiation.4 ) More frequently, given the
difficulties of measuring the true bolometric flux, the flux over a limited range
of wavelengths is measured. If the light from the object has emission or absorption lines, we can measure the redshift, z. If the object is an extended
source rather than a point of light, we can measure its angular diameter, δθ.
One way of using measured properties to assign a distance is the standard
candle method. A standard candle is an object whose luminosity L is known.
For instance, if some class of astronomical object had luminosities which were
the same throughout all of space-time, they would act as excellent standard
candles – if their unique luminosity L were known. If you know, by some
means or other, the luminosity of an object, then you can use its measured
flux f to define a function called the luminosity distance:
dL ≡
4
Ã
L
4πf
!1/2
.
(7.21)
As expressed more poetically in an anonymous limerick: “Oh, Langley devised the
bolometer: / It’s really a kind of thermometer / Which measures the heat / From a polar
bear’s feet / At a distance of half a kilometer.”
7.2. LUMINOSITY DISTANCE
133
Figure 7.1: An observer at the origin observes a standard candle, of known
luminosity L, at comoving coordinate location (r, θ, φ).
The function dL is called a “distance” because its dimensionality is that of a
distance, and because it is what the proper distance to the standard candle
would be if the universe were static and Euclidean. In a static Euclidean
universe, the propagation of light follows the inverse square law f = L/[4πd 2 ].
Suppose, though, that you are in a universe described by a RobertsonWalker metric (equation 3.25):
ds2 = −c2 dt2 + a(t)2 [dr2 + Sκ (r)2 dΩ2 ] ,
with


 R0 sin(r/R0 )
(κ = +1)
(κ = 0)
Sκ (r) =  r

R0 sinh(r/R0 ) (κ = −1) .
(7.22)
(7.23)
You are at the origin. At the present moment, t = t0 , you see light that was
emitted by a standard candle at comoving coordinate location (r, θ, φ) at a
time te (see Figure 7.1). The photons which were emitted at time te are, at
the present moment, spread over a sphere of proper radius dp (t0 ) = r and
proper surface area Ap (t0 ). If space is flat (κ = 0), then the proper area
134
CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
of the sphere is given by the Euclidean relation Ap (t0 ) = 4πdp (t0 )2 = 4πr2 .
More generally, however,
Ap (t0 ) = 4πSκ (r)2 .
(7.24)
When space is positively curved, Ap (t0 ) < 4πr2 , and the photons are spread
over a smaller area than they would be in flat space. When space is negatively
curved, Ap (t0 ) > 4πr2 , and photons are spread over a larger area than they
would be in flat space.
In addition to these geometric effects, which would apply even in a static
universe, the expansion of the universe causes the observed flux of light from
a standard candle of redshift z to be decreased by a factor of (1 + z)−2 . First,
the expansion of the universe causes the energy of each photon from the
standard candle to decrease. If a photon starts with an energy Ee = hc/λe
when the scale factor is a(te ), by the time we observe it, when the scale factor
is a(t0 ) = 1, the wavelength will have grown to
λ0 =
1
λe = (1 + z)λe ,
a(te )
(7.25)
and the energy will have fallen to
E0 =
Ee
.
1+z
(7.26)
Second, thanks to the expansion of the universe, the time between photon
detections will be greater. If two photons are emitted in the same direction
separated by a time interval δte , the proper distance between them will initially be c(δte ); by the time we detect the photons at time t0 , the proper
distance between them will be stretched to c(δte )(1 + z), and we will detect
them separated by a time interval δt0 = δte (1 + z).
The net result is that in an expanding, spatially curved universe, the
relation between the observed flux f and the luminosity L of a distant light
source is
L
f=
,
(7.27)
4πSκ (r)2 (1 + z)2
and the luminosity distance is
dL = Sκ (r)(1 + z) .
(7.28)
135
7.2. LUMINOSITY DISTANCE
(H0/c) dL
15
Λ−only
Benchmark
10
5
0
matter−only
0
2
z
4
6
Figure 7.2: The luminosity distance of a standard candle with observed redshift z. The bold solid line gives the result for the Benchmark Model, the
dot-dash line for a flat, lambda-only universe, and the dotted line for a flat,
matter-only universe.
The available evidence indicates that our universe is nearly flat, with a radius
of curvature R0 which is larger than the current horizon distance dhor (t0 ).
Objects with finite redshift are at proper distances smaller than the horizon
distance, and hence smaller than the radius of curvature. Thus, it is safe to
make the approximation r ¿ R0 , implying Sκ (r) ≈ r. With our assumption
that space is very close to being flat, the relation between the luminosity
distance and the current proper distance becomes very simple:
dL = r(1 + z) = dp (t0 )(1 + z)
[κ = 0] .
(7.29)
Thus, even if space is perfectly flat, if you estimate the distance to a standard
candle by using a naı̈ve inverse square law, you will overestimate the actual
proper distance by a factor (1 + z), where z is the standard candle’s redshift.
Figure 7.2 shows the luminosity distance dL as a function of redshift for
the Benchmark Model, and for two other flat universes, one dominated by
matter and one dominated by a cosmological constant. When z ¿ 1, the
136
CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
current proper distance may be approximated as
1 + q0
c
z 1−
z
dP (t0 ) ≈
H0
2
µ
¶
.
(7.30)
In a universe which is nearly flat, the luminosity distance may thus be approximated as
c
c
1 + q0
1 − q0
dL ≈
z (1 + z) ≈
z
z 1−
z 1+
H0
2
H0
2
¶
µ
µ
Note that in the limit z → 0,
dp (t0 ) ≈ dL ≈
c
z.
H0
¶
.
(7.31)
(7.32)
In a universe described by the Robertson-Walker metric, the luminosity distance is a good approximation to the current proper distance for objects with
small redshifts.
7.3
Angular-diameter distance
The luminosity distance dL is not the only distance measure that can be
computed using the observable properties of cosmological objects. Suppose
that instead of a standard candle, you observed a standard yardstick. A
standard yardstick is an object whose proper length ` is known. In most
cases, it is convenient to chose as your yardstick an object which is tightly
bound together, by gravity or duct tape or some other influence, and hence
is not expanding along with the universe as a whole.
Suppose a yardstick of constant proper length ` is aligned perpendicular
to your line of sight, as shown in Figure 7.3. You measure an angular distance
δθ between the ends of the yardstick, and a redshift z for the light which the
yardstick emits. If δθ ¿ 1, and if you know the length ` of the yardstick,
you can compute a distance to the yardstick using the small-angle formula
dA ≡
`
.
δθ
(7.33)
This function of ` and δθ is called the angular-diameter distance. The
angular-diameter distance is equal to the proper distance to the yardstick
if the universe is static and Euclidean.
7.3. ANGULAR-DIAMETER DISTANCE
137
Figure 7.3: An observer at the origin observes a standard yardstick, of known
proper length `, at comoving coordinate distance r.
In general, though, if the universe is expanding or curved, the angulardiameter distance will not be equal to the current proper distance. Suppose
you are in a universe described by the Robertson-Walker metric given in
equation (7.22). Choose your comoving coordinate system so that you are
at the origin. The yardstick is at a comoving coordinate distance r. At a
time te , the yardstick emitted the light which you observe at time t0 . The
comoving coordinates of the two ends of the yardstick, at the time the light
was emitted, were (r, θ1 , φ) and (r, θ2 , φ). As the light from the yardstick
moves toward the origin, it travels along geodesics with θ = constant and
φ = constant. Thus, the angular size which you measure for the yardstick
will be δθ = θ2 − θ1 . The distance ds between the two ends of the yardstick,
measured at the time te when the light was emitted, can be found from the
Robertson-Walker metric:
ds = a(te )Sκ (r)δθ .
(7.34)
However, for a standard yardstick whose length ` is known, we can set ds = `,
and thus find that
Sκ (r)δθ
.
(7.35)
` = a(te )Sκ (r)δθ =
1+z
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
Thus, the angular-diameter distance dA to a standard yardstick is
dA ≡
Sκ (r)
`
=
.
δθ
1+z
(7.36)
Comparison with equation (7.28) shows that the relation between the angulardiameter distance and the luminosity distance is
dA =
dL
.
(1 + z)2
(7.37)
Thus, if you observe an object which is both a standard candle and a standard
yardstick, the angular-diameter distance which you compute for the object
will be smaller than the luminosity distance. Moreover, if the universe is
spatially flat,
dL
[κ = 0] .
(7.38)
1+z
In a flat universe, therefore, if you compute the angular-diameter distance dA
of a standard yardstick, it isn’t equal to the current proper distance dp (t0 );
rather, it is equal to the proper distance at the time the light from the object
was emitted: dA = dp (t0 )/(1 + z) = dp (te ).
Figure 7.4 shows the angular-diameter distance dA for the Benchmark
Model, and for two other spatially flat universes, one dominated by matter
and one dominated by a cosmological constant. (Since dA is, for these flat
universes, equal to dp (te ), Figure 7.4 is simply a replotting of of the lower
panel in Figure 6.6.) When z ¿ 1, the approximate value of dA is given by
the expansion
¶
µ
3 + q0
c
z .
(7.39)
z 1−
dA ≈
H0
2
Thus, comparing equations (7.30), (7.31), and (7.39), we find that in the
limit z → 0, dA ≈ dL ≈ dp (t0 ) ≈ (c/H0 )z. However, the state of affairs
is very different in the limit z → ∞. In models with a finite horizon size,
dp (t0 ) → dhor (t0 ) as z → ∞. The luminosity distance to highly redshifted
objects, in this case, diverges as z → ∞, with
dA (1 + z) = dp (t0 ) =
dL (z → ∞) ≈ zdhor (t0 ) .
(7.40)
However, the angular-diameter distance to highly redshifted objects approaches
zero as z → ∞, with
dhor (t0 )
dA (z → ∞) ≈
.
(7.41)
z
139
7.3. ANGULAR-DIAMETER DISTANCE
1
(H0/c) dA
.8
Λ−only
.6
.4
Benchmark
.2
0
matter−only
0
2
z
4
6
Figure 7.4: The angular-diameter distance for a standard yardstick with
observed redshift z. The bold solid line gives the result for the Benchmark
Model, the dot-dash line for a flat, lambda-only universe, and the dotted line
for a flat, matter-only universe.
140
CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
In model universes other than the lambda-only model, the angular-diameter
distance dA has a maximum for standard yardsticks at some critical redshift
zc . (For the Benchmark Model, zc = 1.6, where dA (max) = 0.41c/H0 =
1800 Mpc.) This means that if the universe were full of glow-in-the-dark
yardsticks, all of the same size `, their angular size δθ would decrease with
redshift out to z = zc , but then would increase at larger redshifts. The sky
would be full of big, faint, redshifted yardsticks.
In principle, standard yardsticks can be used to determine H0 . To begin
with, identify a population of standard yardsticks (objects whose physical
size ` is known). Then, measure the redshift z and angular size δθ of each
standard yardstick. Compute the angular-diameter distance dA = `/δθ for
each standard yardstick. Plot cz versus dA , and the slope of the relation,
in the limit z → 0, will give you H0 . In addition, if you have measured the
angular size δθ for standard candles at z ∼ zc , the shape of the cz versus dA
plot can be used to determine further cosmological parameters. If you simply
want a kinematic description, you can estimate q0 by fitting equation (7.39)
to the data. If you are confident that the universe is dominated by matter and
a cosmological constant, you can see which values of Ωm,0 and ΩΛ,0 provide
the best fit to the observed data.
In practice, the use of standard yardsticks to determine cosmological parameters has long been plagued with observational difficulties. For instance,
a standard yardstick must have an angular size large enough to be resolved
by your telescope. A yardstick of physical size ` will have its angular size δθ
minimized when it is at the critical redshift zc . For the Benchmark Model,
Ã
`
`
`
δθ(min) =
=
≈ 0.1 arcsec
dA (max)
1800 Mpc
1 kpc
!
.
(7.42)
Both galaxies and clusters of galaxies are large enough to be useful standard
candles. Unfortunately for cosmologists, galaxies and clusters of galaxies do
not have sharply defined edges, so assigning a particular angular size δθ, and
a corresponding physical size `, to these objects is a somewhat tricky task.
Moreover, galaxies and clusters of galaxies are not isolated, rigid yardsticks
of fixed length. Galaxies tend to become larger with time as they undergo
mergers with their neighbors. Clusters, too, tend to become larger with
time, as galaxies fall into them. (Eventually, our Local Group will fall into
the Virgo cluster.) Correcting for these evolutionary trends is a difficult task.
Given the difficulties involved in using standard yardsticks to determine
cosmological parameters, more attention has been focused, in recent years,
7.4. STANDARD CANDLES & H0
141
on the use of standard candles. Let’s first look, therefore, at how standard
candles can be used to determine H0 , then focus on how they can be used to
determine the acceleration of the universe.
7.4
Standard candles & the Hubble constant
Using standard candles to determine the Hubble constant has a long and
honorable history; it’s the method used by Hubble himself. The recipe for
finding the Hubble constant is a simple one:
• Identify a population of standard candles with luminosity L.
• Measure the redshift z and flux f for each standard candle.
• Compute dL = (L/4πf )1/2 for each standard candle.
• Plot cz versus dL .
• Measure the slope of the cz versus dL relation when z ¿ 1; this gives
H0 .
As with the apocryphal recipe for rabbit stew which begins “First catch your
rabbit,” the hardest step is the first one. A good standard candle is hard to
find. For cosmological purposes, a standard candle should be bright enough
to be detected at large redshifts. It should also have a luminosity which is
well determined.5
One time-honored variety of standard candle is the class of Cepheid variable stars. Cepheids, as they are known, are highly luminous supergiant stars,
with mean luminosities lying in the range L̄ = 400 → 40,000 L¯ . Cepheids
are pulsationally unstable. As they pulsate radially, their luminosity varies
in response, partially due to the change in their surface area, and partially
due to the changes in the surface temperature as the star pulsates. The
pulsational periods, as reflected in the observed brightness variations of the
star, lie in the range P = 1.5 → 60 days.
5
A useful cautionary tale, in this regard, is the saga of Edwin Hubble. In the 1929 paper
which first demonstrated that dL ∝ z when z ¿ 1, Hubble underestimated the luminosity
distances to galaxies by a factor of ∼ 7 because he underestimated the luminosity of his
standard candles by a factor of ∼ 49.
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
On the face of it, Cepheids don’t seem sufficiently standardized to be
standard candles; their mean luminosities range over two orders of magnitude. How can you tell whether you are looking at an intrinsically faint
Cepheid (L ≈ 400 L¯ ) or at an intrinsically bright Cepheid (L ≈ 40,000 L¯ )
ten times farther away? The key to calibrating Cepheids was discovered by
Henrietta Leavitt, at Harvard College Observatory. In the years prior to
World War I, Leavitt was studying variable stars in the Large and Small
Magellanic Clouds, a pair of relatively small satellite galaxies orbiting our
own Galaxy. For each Cepheid in the Small Magellanic Cloud (SMC), she
measured the period P by finding the time between maxima in the observed
brightness, and found the mean flux f¯, averaged over one complete period.
She noted that there was a clear relation between P and f¯, with stars having
the longest period of variability also having the largest flux. Since the depth
of the SMC, front to back, is small compared to its distance from us, she
was justified in assuming that the difference in mean flux for the Cepheids
was due to differences in their mean luminosity, not differences in their luminosity distance. Leavitt had discovered a period – luminosity relation for
Cepheid variable stars. If the same period – luminosity relation holds true
for all Cepheids, in all galaxies, then Cepheids can act as a standard candle.
Suppose, for instance, you find a Cepheid star in the Large Magellanic
Cloud (LMC) and another in M31. They both have a pulsational period of
10 days, so you assume, from the period – luminosity relation, that they have
the same mean luminosity L̄. By careful measurement, you determine that
f¯LMC
= 230 .
f¯M31
(7.43)
Thus, you conclude that the luminosity distance to M31 is greater than that
to the LMC6 by a factor
dL (M31)
=
dL (LMC)
Ã
f¯LMC
f¯M31
!1/2
=
√
230 = 15.2 .
(7.44)
Note that if you only know the relative fluxes of the two Cepheids, and not
their luminosity L̄, you will only know the relative distances of M31 and the
6
In practice, given the intrinsic scatter in the period – luminosity relation, and the
inevitable error in measuring fluxes, astronomers would not rely on a single Cepheid in
each galaxy. Rather, they would measure f¯ and P for as many Cepheids as possible in
each galaxy, then find the ratio of luminosity distances that would make the period –
luminosity relations for the two galaxies coincide.
7.4. STANDARD CANDLES & H0
143
LMC. To fix an absolute distance to M31, to the LMC, and to other galaxies
containing Cepheids, you need to know the luminosity L̄ for a Cepheid of a
given period P . If, for instance, you could measure the parallax distance dπ to
a Cepheid within our own Galaxy, you could then compute its luminosity L̄ =
4πd2π f¯, and use it to normalize the period – luminosity relation for Cepheids.7
Unfortunately, Cepheids are rare stars; only the very nearest Cepheids in our
Galaxy have had their distances measured with even modest accuracy by the
Hipparcos satellite. The nearest Cepheid is Polaris, as it turns out, at d π =
130±10 pc. The next nearest is probably δ Cephei (the prototype after which
all Cepheids are named), at dπ = 300±50 pc. Future space-based astrometric
observatories, such as the Space Interferometry Mission (SIM), scheduled for
launch in 2009, will allow parallax distances to be measured with an accuracy
greater than that provided by Hipparcos. Until the distances (and hence
the luminosities) of nearby Cepheids are known with this great accuracy,
astronomers must still rely on alternate methods of normalizing the period
– luminosity relation for Cepheids. The most usual method involves finding
the distance to the Large Magellanic Cloud by secondary methods,8 then
using this distance to compute the mean luminosity of the LMC Cepheids.
The current consensus is that the Large Magellanic Cloud has a luminosity
distance dL = 50 ± 3 kpc, implying a distance to M31 of dL = 760 ± 50 kpc.
With the Hubble Space Telescope, the fluxes and periods of Cepheids
can be accurately measured out to luminosity distances of dL ∼ 20 Mpc.
Observation of Cepheid stars in the Virgo cluster of galaxies, for instance,
has yielded a distance dL (Virgo) = 300 dL (LMC) = 15 Mpc. One of the
motivating reasons for building the Hubble Space Telescope in the first place
was to use Cepheids to determine H0 . The net result of the Hubble Key
Project to measure H0 is displayed in Figure 2.5, showing that the Cepheid
data are best fit with a Hubble constant of H0 = 75 ± 8 km s−1 Mpc−1 .
There is a hidden difficulty involved in using Cepheid stars to determine
H0 . Cepheids can take you out only to a distance dL ∼ 20 Mpc; on this scale,
the universe cannot be assumed to be homogeneous and isotropic. In fact, the
Local Group is gravitationally attracted toward the Virgo cluster, causing it
to have a peculiar motion in that direction. It is estimated, from dynamical
models, that the recession velocity cz which we measure for the Virgo cluster
7
Within our Galaxy, which is not expanding, the parallax distance, the luminosity
distance, and the proper distance are identical.
8
A good review of these methods, and the distances they yield, is given by van den
Bergh (2000).
144
CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
is 250 km s−1 less than it would be if the universe were perfectly homogeneous.
The plot of cz versus dL given in Figure 2.5 uses recession velocities which
are corrected for this “Virgocentric flow”, as it is called.
7.5
Standard candles & the accelerating universe
To determine the value of H0 without having to worry about Virgocentric flow
and other peculiar velocities, we need to determine the luminosity distance
to standard candles with dL > 100 Mpc, or z > 0.02. To determine the value
of q0 , we need to view standard candles for which the relation between dL
and z deviates significantly from the linear relation which holds true at lower
redshifts. In terms of H0 and q0 , the luminosity distance at small redshift is
dL ≈
c
1 − q0
z 1+
z .
H0
2
·
¸
(7.45)
At a redshift z = 0.2, for instance, the luminosity distance dL in the Benchmark Model (with q0 = −0.55) is 5% larger than dL in an empty universe
(with q0 = 0).9
For a standard candle to be seen at dL > 100 Mpc (to determine H0 with
minimal effects from peculiar velocity) or at dL > 1000 Mpc (to determine
q0 ), it must be very luminous. Initial attempts to find a highly luminous
standard candle focused on using entire galaxies as standard candles. This
attempt foundered on the lack of standardization among galaxies. Not only
do galaxies have a wide range of luminosities at the present moment, but any
individual galaxy has a luminosity which evolves significantly with time. For
instance, an isolated galaxy, after an initial outburst of star formation, will
fade gradually with time, as its stars exhaust their nuclear fuel and become
dim stellar remnants. A galaxy in a rich cluster, by contrast, can actually
become more luminous with time, as it “cannibalizes” smaller galaxies by
merging with them. For any particular galaxy, it’s difficult to tell which
effect dominates. Since the luminosity evolution of galaxies is imperfectly
understood, they aren’t particularly suitable for use as standard candles.
9
If you think, optimistically, that you can determine luminosity distances with an
accuracy much better than 5%, then you won’t have to go as deep into space to determine
q0 accurately.
7.5. STANDARD CANDLES & ACCELERATION
145
In recent years, the standard candle of choice among cosmologists has
been type Ia supernovae. A supernova may be loosely defined as an exploding
star. Early in the history of supernova studies, when little was known about
their underlying physics, supernovae were divided into two classes, on the
basis of their spectra. Type I supernovae contain no hydrogen absorption
lines in their spectra; type II supernovae contain strong hydrogen absorption
lines. Gradually, it was realized that all type II supernovae are the same
species of beast; they are massive stars (M > 8 M¯ ) whose cores collapse to
form a black hole or neutron star when their nuclear fuel is exhausted. During
the rapid collapse of the core, the outer layers of the star are thrown off into
space. Type I supernovae are actually two separate species, which are called
type Ia and type Ib. Type Ib supernovae, it is thought, are massive stars
whose cores collapse after the hydrogen-rich outer layers of the star have been
blown away in strong stellar winds. Thus, type Ib and type II supernovae are
driven by very similar mechanisms – their differences are superficial, in the
most literal sense. Type Ia supernovae, however, are something completely
different. They occur in close binary systems where one of the two stars in
the system is a white dwarf; that is, a stellar remnant which is supported
against gravity by electron degeneracy pressure. The transfer of mass from
the companion star to the white dwarf eventually nudges the white dwarf
over the Chandrasekhar limit of 1.4 M¯ ; this is the maximum mass at which
the electron degeneracy pressure can support a white dwarf against its own
self-gravity. When the Chandrasekhar limit is exceeded, the white dwarf
starts to collapse until its increased density triggers a runaway nuclear fusion
reaction. The entire white dwarf becomes a fusion bomb, blowing itself to
smithereens; unlike type II supernovae, type Ia supernovae do not leave a
condensed stellar remnant behind.
Within our Galaxy, type Ia supernovae occur roughly once per century,
on average. Although type Ia supernovae are not frequent occurrences locally, they are extraordinary luminous, and hence can be seen to large distances. The luminosity of an average type Ia supernova, at peak brightness,
is L = 4×109 L¯ ; that’s 100,000 times more luminous than even the brightest
Cepheid. For a few days, a type Ia supernova in a moderately bright galaxy
can outshine all the other stars in the galaxy combined. Since moderately
bright galaxies can be seen at z ∼ 1, this means that type Ia supernovae
can also be seen at z ∼ 1. Not only are type Ia supernovae bright standard
candles, they are also reasonably standardized standard candles. Consider
type Ia supernovae in the Virgo cluster. Although there’s only one type Ia
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
supernova per century in our own Galaxy, the total luminosity of the Virgo
cluster is a few hundred times that of our Galaxy. Thus, every year you can
expect a few type Ia supernovae to go off in the Virgo cluster. Several type
Ia supernovae have been observed in the Virgo cluster in the recent past, and
have been found to have similar fluxes at maximum brightness.
So far, type Ia supernovae sound like ideal standard candles; very luminous and very standardized. There’s one complication, however. Observation of supernovae in galaxies whose distances have been well determined by
Cepheids reveal that type Ia supernovae do not have identical luminosities.
Instead of all having L = 4 × 109 L¯ , their peak luminosities lie in the fairly
broad range L ≈ 3 → 5 × 109 L¯ . However, it has also been noted that the
peak luminosity of a type Ia supernova is tightly correlated with the shape
of its light curve. Type Ia supernovae with luminosities the shoot up rapidly
and decline rapidly are less luminous than average at their peak; supernovae
with luminosities which rise and fall in a more leisurely manner are more
luminous than average. Thus, just as the period of a Cepheid tells you its
luminosity, the rise and fall time of a type Ia supernova tells you its peak
luminosity.
Recently, two research teams, the “Supernova Cosmology Project” and
the “High-z Supernova Search Team”, have been conducting searches for
supernovae in distant galaxies. They have used the observed light curves and
redshifts of type Ia supernovae to measure cosmological parameters. First, by
observing type Ia supernovae at z ∼ 0.1, the value of H0 can be determined.
The results of the different groups are in reasonable agreement with each
other. If the distance to the Virgo cluster is pegged at dL = 15 Mpc, as
indicated by the Cepheid results, then the observed supernovae fluxes and
redshifts are consistent with H0 = 70 ± 7 km s−1 Mpc−1 , the value of the
Hubble constant which I have adopted in this text.
In addition, the supernova groups have been attempting to measure the
acceleration (or deceleration) of the universe by observing type Ia supernovae
at higher redshift. To present the most recent supernova results to you, I will
have to introduce the “magnitude” system used by astronomers to express
fluxes and luminosities. The magnitude system, like much else in astronomy,
has its roots in ancient Greece. The Greek astronomer Hipparchus, in the
second century BC, divided the stars into six classes, according to their apparent brightness. The brightest stars were of “first magnitude”, the faintest
stars visible to the naked eye were of “sixth magnitude”, and intermediate
stars were ranked as second, third, fourth, and fifth magnitude. Long after
7.5. STANDARD CANDLES & ACCELERATION
147
the time of Hipparchus, it was realized that the response of the human eye
is roughly logarithmic, and that stars of the first magnitude have fluxes (at
visible wavelengths) about 100 times greater than stars of the sixth magnitude. On the basis of this realization, the magnitude system was placed on
a more rigorous mathematical basis.
Nowadays, the bolometric apparent magnitude of a light source is defined
in terms of the source’s bolometric flux as
m ≡ −2.5 log10 (f /fx ) ,
(7.46)
where the reference flux fx is set at the value fx = 2.53 × 10−8 watt m−2 .
Thanks to the negative sign in the definition, a small value of m corresponds
to a large flux f . For instance, the flux of sunlight at the Earth’s location
is f = 1367 watts m−2 ; the Sun thus has a bolometric apparent magnitude
of m = −26.8. The choice of reference flux fx constitutes a tip of the hat
to Hipparchus, since for stars visible to the naked eye it typically yields
0 < m < 6.
The bolometric absolute magnitude of a light source is defined as the
apparent magnitude that it would have if it were at a luminosity distance of
dL = 10 pc. Thus, a light source with luminosity L has a bolometric absolute
magnitude
M ≡ −2.5 log10 (L/Lx ) ,
(7.47)
where the reference luminosity is Lx = 78.7 L¯ , since that is the luminosity
of an object which produces a flux fx = 2.53 × 10−8 watt m−2 when viewed
from a distance of 10 parsecs. The bolometric absolute magnitude of the Sun
is thus M = 4.74. Although the system of apparent and absolute magnitudes
seems strange to the uninitiated, the apparent magnitude is really nothing
more than a logarithmic measure of the flux, and the absolute magnitude is
a logarithmic measure of the luminosity.
Given the definitions of apparent and absolute magnitude, the relation
between an object’s apparent magnitude and its absolute magnitude can be
written in the form
Ã
!
dL
M = m − 5 log10
,
(7.48)
10 pc
where dL is the luminosity distance to the light source. If the luminosity
distance is given in units of megaparsecs, this relation becomes
M = m − 5 log10
Ã
!
dL
− 25 .
1 Mpc
(7.49)
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
Since astronomers frequently quote fluxes and luminosities in terms of apparent and absolute magnitudes, they find it convenient to quote luminosity
distances in terms of the distance modulus to a light source. The distance
modulus is defined as m − M , and is related to the luminosity distance by
the relation
Ã
!
dL
m − M = 5 log10
+ 25 .
(7.50)
1 Mpc
The distance modulus of the Large Magellanic Cloud, for instance, at dL =
0.050 Mpc, is m − M = 18.5. The distance modulus of the Virgo cluster, at
dL = 15 Mpc, is m − M = 30.9. When z ¿ 1, the luminosity distance to a
light source is
µ
¶
1 − q0
c
z 1+
z .
(7.51)
dL ≈
H0
2
Substituting this relation into equation (7.50), we have an equation which
gives the relation between distance modulus and redshift:
m − M ≈ 43.17 − 5 log10
Ã
H0
70 km s−1 Mpc−1
!
+ 5 log10 z + 1.086(1 − q0 )z .
(7.52)
For a population of standard candles with known luminosity L (and hence
of known bolometric absolute magnitude M ), you measure the flux f (or
equivalently, the bolometric apparent magnitude m) and the redshift z. In
the limit z → 0, a plot of m − M versus log 10 z gives a straight line whose
amplitude at a given value of z tells you the value of H0 . At slightly larger
values of z, the deviation of the plot from a straight line tells you the value
of q0 . At a given value of z, an accelerating universe (with q0 < 0) yields
standard candles with a smaller flux than would a decelerating universe (with
q0 > 0).
The upper panel of Figure 7.5 shows the plot of distance modulus versus
redshift for the combined supernova samples of the High-z Supernova Search
Team (given by the filled circles) and the Supernova Cosmology Project
(given by the open circles). The observational results are compared to the
expected results for three model universes. One universe is flat, and contains
nothing but matter (Ωm,0 = 1, q0 = 0.5). The second is negatively curved,
and contains nothing but matter (Ωm,0 = 0.3, q0 = 0.15). The third is
flat, and contains both matter and a cosmological constant (Ωm,0 = 0.3,
ΩΛ,0 = 0.7, q0 = −0.55). The data are best fitted by the third of the
models – which is, in fact, our Benchmark Model. The bottom panel of
7.5. STANDARD CANDLES & ACCELERATION
149
Figure 7.5: Distance modulus versus redshift for type Ia supernovae from the
Supernova Cosmology Project (Perlmutter et al. 1999, ApJ, 517, 565) and
the High-z Supernova Search Team (Riess et al. 1998, AJ, 116, 1009). The
bottom panel shows the difference between the data and the predictions of a
negatively curved Ωm,0 = 0.3 model (from Riess 2000, PASP, 112, 1284).
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
Figure 7.5 shows this result more clearly. It shows the difference between
the data and the predictions of the negatively curved, matter-only model.
The conclusion that the universe is accelerating derives from the observation
that the supernovae seen at z ∼ 0.5 are, on average, about 0.25 magnitudes
fainter than they would in a decelerating universe with Ωm,0 = 0.3 and no
cosmological constant.
The supernova data extend out to z ∼ 1; this is beyond the range where an
expansion in terms of H0 and q0 is adequate to describe the scale factor a(t).
Thus, the two supernova teams customarily describe their results in terms of
a model universe which contains both matter and a cosmological constant.
After choosing values of Ωm,0 and ΩΛ,0 , they compute the expected relation
between m − M and z, and compare it to the observed data. The results
of fitting these model universes are given in Figure 7.6. The ovals drawn on
Figure 7.6 enclose those values of Ωm,0 and ΩΛ,0 which give the best fit to
the supernova data. The results of the two teams (the solid ovals and dotted
ovals) give very similar results. Three concentric ovals are shown for each
team’s result; they correspond to 1σ, 2σ, and 3σ confidence intervals, with
the inner oval representing the highest probability. The best fitting models
lie along the line 0.8Ωm,0 − 0.6ΩΛ,0 ≈ −0.2. Note that decelerating universes
(with q0 > 0) can be strongly excluded by the data, as can Big Crunch
universes (labeled ‘Recollapses’ in Figure 7.6), and Big Bounce universes
(labeled ‘No Big Bang’ in Figure 7.6). The supernova data are consistent
with negative curvature (labeled ‘Open’ in Figure 7.6), positive curvature
(labeled ‘Closed’ in Figure 7.6), or with a universe which is spatially flat.
The results of the supernova teams made headlines when they were first
announced; the discovery of the accelerating universe was named by Science
magazine as the ‘Scientific Breakthrough of the Year’ for 1998. It is prudent
to remember, however, that all the hoopla about the accelerating universe is
based on the observation that type Ia supernova at z ∼ 0.5 and beyond have
somewhat lower fluxes (by about 25%) than they would have in a decelerating
universe. There are other reasons why their fluxes might be low. For instance,
if type Ia supernovae were intrinsically less luminous at z ∼ 0.5 than at z ∼ 0,
that could explain their low fluxes. (If a typical supernova at z ∼ 0.5 had L =
3 × 109 L¯ rather than 4 × 109 L¯ , that would explain their observed dimness,
without the need to invoke a cosmological constant. Conversely, if the typical
supernova at z ∼ 0.5 had L = 5 × 109 L¯ rather than 4 × 109 L¯ , that
would require an even larger cosmological constant to explain their observed
dimness.) However, the other properties of type Ia supernovae, such as their
7.5. STANDARD CANDLES & ACCELERATION
151
Figure 7.6: The values of Ωm,0 (horizontal axis) and ΩΛ,0 (vertical axis) which
best fit the data shown in Figure 7.5. The solid ovals show the best-fitting
values for the High-z Supernova Search Team data; the dotted ovals show
the best-fitting values for the Supernova Cosmology Project data (from Riess
2000, PASP, 112, 1284).
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
spectra, don’t seem to evolve with time, so why should their luminosity?
Perhaps the fluxes of supernovae at z ∼ 0.5 are low because some of their
light is scattered or absorbed by intervening dust. However, dust tends to
scatter some wavelengths of light more than others. This would change the
shape of the spectrum of distant type Ia supernovae, but no dependence of
spectral shape on redshift is observed.
In sum, the supernova results of Figure 7.6 provide persuasive (but, given
the caveats, not absolutely compelling) evidence for an accelerating universe.
We will see in future chapters how additional observational evidence interlocks with the supernova results to suggest that we live in a nearly flat
accelerating universe with Ωm,0 ≈ 0.3 and ΩΛ,0 ≈ 0.7.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Liddle (1999), ch. 6: The relation among H0 , q0 , Ω0 , and Λ.
Narlikar (2002), ch. 9, 10: Local observations (z < 0.1) and more distant
observations (z > 0.1) of the universe, and what they tell us about
cosmological parameters.
Peacock (1999), ch. 5: A review of distance measures used in cosmology.
Rich (2001), ch. 5.2: A summary of the supernova Ia results.
Problems
(7.1) Suppose that a polar bear’s foot has a luminosity of L = 10 watts.
What is the bolometric absolute magnitude of the bear’s foot? What is
the bolometric apparent magnitude of the foot at a luminosity distance
of dL = 0.5 km? If a bolometer can detect the bear’s foot at a maximum
luminosity distance of dL = 0.5 km, what is the maximum luminosity
distance at which it could detect the Sun? What is the maximum
luminosity distance at which it could detect a supernova with L =
4 × 109 L¯ ?
7.5. STANDARD CANDLES & ACCELERATION
153
(7.2) Suppose that a polar bear’s foot has a diameter of ` = 0.16 m. What
is the angular size δθ of the foot at an angular-diameter distance of
dA = 0.5 km? In the Benchmark Model, what is the minimum possible
angular size of the polar bear’s foot?
(7.3) Suppose that you are in a spatially flat universe containing a single
component with a unique equation-of-state parameter w. What are
the current proper distance dP (t0 ), the luminosity distance dL and the
angular-diameter distance dA as a function of z and w? At what redshift
will dA have a maximum value? What will this maximum value be, in
units of the Hubble distance?
(7.4) Verify that equation (7.52) is correct in the limit of small z. (You will
probably want to use the relation log10 (1 + x) ≈ 0.4343 ln(1 + x) ≈
0.4343x in the limit |x| ¿ 1.)
(7.5) The surface brightness Σ of an astronomical object is defined as its
observed flux divided by its observed angular area; thus, Σ ∝ f /(δθ)2 .
For a class of objects which are both standard candles and standard
yardsticks, what is Σ as a function of redshift? Would observing the
surface brightness of this class of objects be a useful way of determining
the value of the deceleration parameter q0 ? Why or why not?
(7.6) You observe a quasar at a redshift z = 5.0, and determine that the
observed flux of light from the quasar varies on a timescale δt0 = 3 days.
If the observed variation in flux is due to a variation in the intrinsic
luminosity of the quasar, what was the variation timescale δte at the
time the light was emitted? For the light from the quasar to vary on a
timescale δte , the bulk of the light must come from a region of physical
size R ≤ Rmax = c(δte ). What is Rmax for the observed quasar? What
is the angular size of Rmax in the Benchmark Model?
(7.7) Derive the relation Ap (t0 ) = 4πSκ (r)2 , as given in equation (7.24),
starting from the Robertson-Walker metric of equation (7.22).
(7.8) A spatially flat universe contains a single component with equationof-state parameter w. In this universe, standard candles of luminosity
L are distributed homogeneously in space. The number density of
the standard candles is n0 at t = t0 , and the standard candles are
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CHAPTER 7. MEASURING COSMOLOGICAL PARAMETERS
neither created nor destroyed. Show that the observed flux from a
single standard candle at redshift z is
f (z) =
h
i
L(1 + 3w)2
1
−(1+3w)/2 −2
1
−
(1
+
z)
16π(c/H0 )2 (1 + z)2
(7.53)
when w 6= −1/3. What is the corresponding relation when w = −1/3?
Show that the observed intensity (that is, the power per unit area per
steradian of sky) from standard candles with redshifts in the range
z → z + dz is
dJ(z) =
n0 L(c/H0 )
(1 + z)−(7+3w)/2 dz .
4π
(7.54)
What will be the total intensity J of all standard candles integrated
over all redshifts? Explain why the night sky is of finite brightness even
in universes with w ≤ −1/3, which have an infinite horizon distance.
Chapter 8
Dark Matter
Cosmologists, over the years, have dedicated a large amount of time and
effort to determining the matter density of the universe. There are many
reasons for this obsession. First, the density parameter in matter, Ωm,0 , is
important in determining the spatial curvature and expansion rate of the
universe. Even if the cosmological constant is non-zero, the matter content
of the universe is not negligible today, and was the dominant component in
the fairly recent past. Another reason for wanting to know the matter density
of the universe is to find out what the universe is made of. What fraction
of the density is made of stars, and other familiar types of baryonic matter?
What fraction of the density is made of dark matter? What constitutes the
dark matter – cold stellar remnants, black holes, exotic elementary particles,
or some other substance too dim for us to see? These questions, and others,
have driven astronomers to make a census of the universe, and find out what
types of matter it contains, and in what quantities.
We have already seen, in the previous chapter, one method of putting
limits on Ωm,0 . The apparent magnitude (or flux) of type Ia supernovae as a
function of redshift is consistent with a flat universe having Ωm,0 ≈ 0.3 and
ΩΛ,0 ≈ 0.7. However, neither Ωm,0 nor ΩΛ,0 is individually well-constrained by
the supernova observations. The supernova data are consistent with Ωm,0 = 0
if ΩΛ,0 ≈ 0.4; they are also consistent with Ωm,0 = 1 if ΩΛ,0 ≈ 1.7. In order
to determine Ωm,0 more accurately, we will have to adopt alternate methods
of estimating the matter content of the universe.
155
156
8.1
CHAPTER 8. DARK MATTER
Visible matter
Some types of matter, such as stars, help astronomers to detect them by
broadcasting photons in all directions. Stars primarily emit light in the infrared, visible, and ultraviolet range of the electromagnetic spectrum. Suppose, for instance, you install a B-band filter on your telescope. Such a filter
allows only photons in the wavelength range 4.0 × 10−7 m < λ < 4.9 × 10−7 m
to pass through.1 The “B” in B-band stands for “blue”; however, in addition
to admitting blue light, a B-band filter also lets through violet light. The
Sun’s luminosity in the B band is L¯,B = 4.7 × 1025 watts. 2
In the B band, the total luminosity density of stars within a few hundred
megaparsecs of our Galaxy is
j?,B = 1.2 × 108 L¯,B Mpc−3 .
(8.1)
hM/LB i ≈ 4 M¯ / L¯,B ≈ 170,000 kg watt−1 .
(8.2)
To convert a luminosity density j?,B into a mass density ρ? , we need to know
the mass-to-light ratio for the stars. That is, we need to know how many
kilograms of star, on average, it takes to produce one watt of starlight in
the B band. If all stars were identical to the Sun, we could simply say that
there is one solar mass of stars for each solar luminosity of output power, or
hM/LB i = 1 M¯ / L¯,B . However, stars are not uniform in their properties.
They have a wide range of masses and a wider range of B-band luminosities.
For main sequence stars, powered by hydrogen fusion in their centers, the
mass-to-light ratio ranges from M/LB ∼ 10−3 M¯ / L¯,B for the brightest,
most massive stars (the O stars in the classic OBAFGKM spectral sequence)
to M/LB ∼ 103 M¯ / L¯,B for the dimmest, least massive stars (the M stars).
Thus, the mass-to-light ratio of the stars in a galaxy will depend on the
mix of stars which it contains. As a first guess, let’s suppose that the mix of
stars in the solar neighborhood is not abnormal. Within 1 kiloparsec of the
Sun, the mass-to-light ratio of the stars works out to be
Although a mass-to-light ratio of 170 tons per watt doesn’t seem, at first
glance, like a very high efficiency, you must remember that the mass of a star
includes all the fuel which it will require during its entire lifetime.
1
For comparison, your eyes detect photons in the wavelength range 4 × 10 −7 m < λ <
7 × 10−7 m.
2
This is only 12% of the Sun’s total luminosity. About 6% of the luminosity is emitted
at ultraviolet wavelengths, and the remaining 82% is emitted at wavelengths too long to
pass through the B filter.
8.1. VISIBLE MATTER
157
If the mass-to-light ratio of the stars within a kiloparsec of us is not
unusually high or low, then the mass density of stars in the universe is
ρ?,0 = hM/LB ij?,B ≈ 5 × 108 M¯ Mpc−3 .
(8.3)
Since the current critical density of the universe is equivalent to a mass
density of ρc,0 = εc,0 /c2 = 1.4×1011 M¯ Mpc−3 , the current density parameter
of stars is
5 × 108 M¯ Mpc−3
ρ?,0
≈
Ω?,0 =
≈ 0.004 .
(8.4)
ρc,0
1.4 × 1011 M¯ Mpc−3
Stars make up less than 1/2% of the density necessary to flatten the universe.
In truth, the number Ω?,0 ≈ 0.004 is not a precisely determined one, largely
because of the uncertainty in the number of low-mass, low-luminosity stars
in galaxies. In our Galaxy, for instance, ∼ 95% of the stellar luminosity
comes from stars more luminous than the Sun, but ∼ 80% of the stellar
mass comes from stars less luminous than the Sun. The density parameter
in stars will be further increased if you include in the category of “stars”
stellar remnants (such as white dwarfs, neutron stars, and black holes) and
brown dwarfs. A brown dwarf is a self-gravitating ball of gas which is too low
in mass to sustain nuclear fusion in its interior. Because brown dwarfs and
isolated cool stellar remnants are difficult to detect, their number density is
not well determined.
Galaxies also contain baryonic matter which is not in the form of stars,
stellar remnants, or brown dwarfs. The interstellar medium contains significant amounts of gas. In our Galaxy and in M31, for instance, the mass
of interstellar gas is roughly equal to 10% of the mass of stars. In irregular
galaxies such as the Magellanic Clouds, the ratio of gas to stars is even higher.
In addition, there is a significant amount of gas between galaxies. Consider
a rich cluster of galaxies such as the Coma cluster, located 100 Mpc from
our Galaxy, in the direction of the constellation Coma Berenices. At visible
wavelengths, as shown in Figure 8.1, most of the light comes from the stars in
the cluster’s galaxies. The Coma cluster contains thousands of galaxies; their
summed luminosity in the B band comes to LComa,B = 8 × 1012 L¯,B . If the
mass-to-light ratio of the stars in the Coma cluster is hM/LB i ≈ 4 M¯ / L¯,B ,
then the total mass of stars in the Coma cluster is MComa,? ≈ 3 × 1013 M¯ .
Although 30 trillion solar masses represents a lot of stars, the stellar mass in
the Coma cluster is small compared to the mass of the hot, intracluster gas
between the galaxies in the cluster. X-ray images, such as the one shown in
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CHAPTER 8. DARK MATTER
Figure 8.1: The Coma cluster as seen in visible light. The image shown is
35 arcminutes across, equivalent to ∼ 1 Mpc at the distance of the Coma
cluster. [From the Digitized Sky Survey, produced at the Space Telescope
Science Institute]
Figure 8.2: The Coma cluster as seen in x-ray light. The scale is the same
as that of the previous image. [From the ROSAT x-ray observatory; courtesy Max-Planck-Institut für extraterrestriche Physik. This figure and the
previous figure were produced by Raymond White, using NASA’s SkyView
facility.]
8.2. DARK MATTER IN GALAXIES
159
Figure 8.2, reveal that hot, low-density gas, with a typical temperature of
T ≈ 1 × 108 K, fills the space between clusters, emitting x-rays with a typical
energy of E ∼ kTgas ∼ 9 keV. The total amount of x-ray emitting gas in
the Coma cluster is estimated to be MComa,gas ≈ 2 × 1014 M¯ , roughly six or
seven times the mass in stars.
As it turns out, the best current limits on the baryon density of the
universe come from the predictions of primordial nucleosynthesis. As we will
see in Chapter 10, the efficiency with which fusion takes place in the early
universe, converting hydrogen into deuterium, helium, lithium, and other
elements, depends on the density of protons and neutrons present. Detailed
studies of the amounts of deuterium and other elements present in primordial
gas clouds indicate that the density parameter of baryonic matter must be
Ωbary,0 = 0.04 ± 0.01 ,
(8.5)
an order of magnitude larger than the density parameter for stars. When
you stare up at the night sky and marvel at the glory of the stars, you are
actually marveling at a minority of the baryonic matter in the universe. Most
of the baryons are too cold to be readily visible (the infrared emitting brown
dwarfs and cold stellar remnants) or too diffuse to be readily visible (the low
density x-ray gas in clusters).
8.2
Dark matter in galaxies
The situation, in fact, is even more extreme than stated in the previous
section. Not only is most of the baryonic matter undetectable by our eyes,
most of the matter is not even baryonic. The majority of the matter in the
universe is nonbaryonic dark matter, which doesn’t absorb, emit, or scatter
light of any wavelength. One way of detecting dark matter is to look for its
gravitational influence on visible matter. A classic method of detecting dark
matter involves looking at the orbital speeds of stars in spiral galaxies such
as our own Galaxy and M31. Spiral galaxies contain flattened disks of stars;
within the disk, stars are on nearly circular orbits around the center of the
galaxy. The Sun, for instance, is on such an orbit – it is R = 8.5 kpc from
the Galactic center, and has an orbital speed of v = 220 km s−1 .
Suppose that a star is on a circular orbit around the center of its galaxy.
If the radius of the orbit is R and the orbital speed is v, then the star
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CHAPTER 8. DARK MATTER
experiences an acceleration
v2
,
(8.6)
R
directed toward the center of the galaxy. If the acceleration is provided by
the gravitational attraction of the galaxy, then
a=
a=
GM (R)
,
R2
(8.7)
where M (R) is the mass contained within a sphere of radius R centered
on the galactic center.3 The relation between v and M is found by setting
equation (8.6) equal to equation (8.7):
GM (R)
v2
=
,
R
R2
or
(8.8)
s
GM (R)
.
(8.9)
R
The surface brightness I of the disk of a spiral galaxy typically falls off
exponentially with distance from the center:
v=
I(R) = I(0) exp(−R/Rs ) ,
(8.10)
with the scale length Rs typically being a few kiloparsecs. For our Galaxy,
Rs ≈ 4 kpc; for M31, a somewhat larger disk galaxy, Rs ≈ 6 kpc. Once you
are a few scale lengths from the center of the spiral galaxy, the mass of stars
inside R becomes essentially constant. Thus, if stars contributed
all, or most,
√
of the mass in a galaxy, the velocity would fall as v ∝ 1/ R√at large radii.
This relation between orbital speed and orbital radius, v ∝ 1/ R, is referred
to as “Keplerian rotation”, since it’s what Kepler found for orbits in the Solar
System, where the mass is strongly concentrated toward the center.4
The orbital speed v of stars within a spiral galaxy can be determined
from observations. Consider a galaxy which has the shape of a thin circular
disk. In general, we won’t be seeing the disk perfectly face-on or edge-on;
3
Equation (8.7) assumes that the mass distribution of the galaxy is spherically symmetric. This is not, strictly speaking, true (the stars in the disk obviously have a flattened
distribution), but the flattening of the galaxy provides only a small correction to the
equation for the gravitational acceleration.
4
99.8% of the Solar System’s mass is contained within the Sun.
8.2. DARK MATTER IN GALAXIES
161
Figure 8.3: An observer sees a disk at an inclination angle i.
we’ll see it at an inclination i, where i is the angle between our line of sight
to the disk and a line perpendicular to the disk (see Figure 8.3). The disk
we see in projection will be elliptical, not circular, with an axis ratio
b/a = cos i .
(8.11)
For example, the galaxy M31 looks extremely elongated as seen from Earth,
with an observed axis ratio b/a = 0.22. This indicates that we are seeing
M31 fairly close to edge-on, with an inclination i = cos−1 (0.22) = 77◦ . By
measuring the redshift of the absorption, or emission, lines in light from the
disk, we can find the radial velocity vr (R) = cz(R) along the apparent long
axis of the galaxy. Since the redshift contains only the component of the
stars’ orbital velocity which lies along the line of sight, the radial velocity
which we measure will be
vr (R) = vgal + v(R) sin i ,
(8.12)
where vgal is the radial velocity of the galaxy as a whole, resulting from the
expansion of the universe, and v(R) is the orbital speed at a distance R from
the center of the disk. We can thus compute the orbital speed v(R) in terms
of observable properties as
v(R) =
vr (R) − vgal
vr (R) − vgal
.
= q
sin i
1 − b2 /a2
(8.13)
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CHAPTER 8. DARK MATTER
Figure 8.4: The orbital speed v as a function of radius in M31. The open
circles show the results of Rubin and Ford (1970, ApJ, 159, 379) at visible
wavelengths; the solid dots with error bars show the results of Roberts and
Whitehurst (1975, ApJ, 201, 327) at radio wavelengths (figure from van den
Bergh, 2000).
The first astronomer to detect the rotation of M31 was Vesto Slipher, in
1914. However, given the difficulty of measuring the spectra at low surface
brightness, the orbital speed v at R > 3Rs = 18 kpc was not accurately
measured until more than half a century later. In 1970, Vera Rubin and Kent
Ford looked at emission lines from regions of hot ionized gas in M31, and were
able to find the orbital speed v(R) out to a radius R = 24 kpc = 4Rs . Their
results, shown as the open circles in Figure 8.4, give no sign of a Keplerian
decrease in the orbital speed. Beyond R = 4Rs , the visible light from M31
was too faint for Rubin and Ford to measure the redshift; as they wrote
in their original paper, “extrapolation beyond that distance is a matter of
taste.” At R > 4Rs , there is still a small amount of atomic hydrogen in
the disk of M31, which can be detected by means of its emission line at
λ = 21 cm. By measuring the redshift of this emission line, M. Roberts
and R. Whitehurst found that the orbital speed stayed at a nearly constant
value of v(R) ≈ 230 km s−1 out to R ≈ 30 kpc ≈ 5Rs , as shown by the solid
dots in Figure 8.4. Since the orbital speed of the stars and gas at large
radii (R > 3Rs ) is greater than it would be if stars and gas were the only
matter present, we deduce the presence of a dark halo within which the visible
stellar disk is embedded. The mass of the dark halo provides the necessary
163
8.2. DARK MATTER IN GALAXIES
gravitational “anchor” to keep the high-speed stars and gas from being flung
out into intergalactic space.
M31 is not a freak; most, if not all, spiral galaxies have comparable dark
halos. For instance, our own Galaxy has an orbital speed which actually
seems to be rising slightly at R > 15 kpc, instead of decreasing in a Keplerian
fashion. Thousands of spiral galaxies have had their orbital velocities v(R)
measured; typically, v is roughly constant at R > Rs . If we approximate the
orbital speed v as being constant with radius, the mass of a spiral galaxy,
including both the luminous disk and the dark halo, can be found from
equation (8.9):
v2R
v
= 9.6 × 1010 M¯
M (R) =
G
220 km s−1
µ
¶2 Ã
R
8.5 kpc
!
.
(8.14)
The values of v and R in the above equation are scaled to the Sun’s location
in our Galaxy. Since our Galaxy’s luminosity in the B band is estimated to
be LGal,B = 2.3 × 1010 L¯,B , this means that the mass-to-light ratio of our
Galaxy, taken as a whole, is
hM/LB iGal ≈ 50 M¯ / L¯,B
Ã
Rhalo
100 kpc
!
,
(8.15)
using v = 220 km s−1 in equation (8.14). The quantity Rhalo is the radius
of the dark halo surrounding the luminous disk of our galaxy. The exact
value of Rhalo is poorly known. At R ≈ 20 kpc, where the last detectable gas
exists in the disk of our Galaxy, the orbital speed shows no sign of a Keplerian
decrease; thus, Rhalo > 20 kpc. A rough estimate of the halo size can be made
by looking at the velocities of the globular clusters and satellite galaxies (such
as the Magellanic Clouds) which orbit our Galaxy. For these hangers-on to
remain gravitationally bound to our Galaxy, the halo must extend as far as
Rhalo ≈ 75 kpc, implying a total mass for our Galaxy of MGal ≈ 8 × 1011 M¯ ,
and a total mass-to-light ratio hM/LB iGal ≈ 40 M/ L¯,B . This mass-to-light
ratio is ten times greater than that of the stars in our Galaxy, implying
that the dark halo is an order of magnitude more massive than the stellar
disk. Some astronomers have speculated that the dark halo is actually four
times larger in radius, with Rhalo ≈ 300 kpc; this would mean that our halo
stretches nearly halfway to M31. With Rhalo ≈ 300 kpc, the mass of our
Galaxy would be MGal ≈ 3 × 1012 M¯ , and the total mass-to-light ratio
would be hM/LB iGal ≈ 150 M¯ / L¯,B .
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CHAPTER 8. DARK MATTER
If our Galaxy is typical in having a dark halo 10 to 40 times more massive
than its stellar component, then the density parameter of galaxies (including
their dark halos) must be
Ωgal,0 = (10 → 40)Ω?,0 ≈ 0.04 → 0.16 .
(8.16)
Although the total density of galaxies is poorly known, given the uncertainty
in the extent of their dark halos, it is likely to be larger than the density of
baryons, Ωbary,0 = 0.04 ± 0.01. Thus, some part of the dark halos of galaxies
is likely to be comprised of nonbaryonic dark matter.
8.3
Dark matter in clusters
The first astronomer to make a compelling case for the existence of large
quantities of dark matter was Fritz Zwicky, in the 1930’s. In studying the
Coma cluster of galaxies (shown in Figure 8.1), he noted that the dispersion in the radial velocity of the cluster’s galaxies was very large – around
1000 km s−1 . The stars and gas visible within the galaxies simply did not provide enough gravitational attraction to hold the cluster together. In order
to keep the galaxies in the Coma cluster from flying off into the surrounding
voids, Zwicky concluded, the cluster must contain a large amount of “dunkle
Materie”, or (translated into English) “dark matter”.5
To follow, at a more mathematical level, Zwicky’s reasoning, let us suppose that a cluster of galaxies is comprised of N galaxies, each of which can
be approximated as a point mass, with a mass mi (i = 1, 2, . . . , N ), a position
~xi , and a velocity ~x˙i . Clusters of galaxies are gravitationally bound objects,
not expanding with the Hubble flow. They are small compared to the horizon size; the radius of the Coma cluster is RComa ≈ 3 Mpc ≈ 0.0002dhor . The
galaxies within a cluster are moving at non-relativistic speeds; the velocity
dispersion within the Coma cluster is σComa ≈ 1000 km s−1 ≈ 0.003c. Because
of these considerations, we can treat the dynamics of the Coma cluster, and
other clusters of galaxies, in a Newtonian manner. The acceleration of the
5
Although Zwicky’s work popularized the phrase “dark matter”, he was not the first
to use it in an astronomical context. For instance, in 1908, Henri Poincaré discussed the
possible existence within our Galaxy of “matière obscure” (translated as “dark matter” in
the standard edition of Poincaré’s works).
8.3. DARK MATTER IN CLUSTERS
165
ith galaxy in the cluster, then, is given by the Newtonian formula
~x¨i = G
X
mj
j6=i
~xj − ~xi
.
|~xj − ~xi |3
(8.17)
Note that equation (8.17) assumes that the cluster is an isolated system,
with the gravitational acceleration due to matter outside the cluster being
negligibly small.
The gravitational potential energy of the system of N galaxies is
W =−
G X mi mj
.
2 i,j |~xj − ~xi |
(8.18)
j6=i
This is the energy that would be required to pull the N galaxies away from
each other so that they would all be at infinite distance from each other.
(The factor of 1/2 in front of the double summation ensures that each pair
of galaxies is only counted once in computing the potential energy.) The
potential energy of the cluster can also be written in the form
W = −α
GM 2
,
rh
(8.19)
where M = mi is the total mass of all the galaxies in the cluster, α is a
numerical factor of order unity which depends on the density profile of the
cluster. and rh is the half-mass radius of the cluster – that is, the radius of a
sphere centered on the cluster’s center of mass and containing a mass M/2.
For observed clusters of galaxies, it is found that α ≈ 0.4 gives a good fit to
the potential energy.
The kinetic energy associated with the relative motion of the galaxies in
the cluster is
1X
K=
mi |~x˙i |2 .
(8.20)
2 i
P
The kinetic energy K can also be written in the form
1
K = M hv 2 i ,
2
where
hv 2 i ≡
1 X
mi |~x˙i |2
M i
(8.21)
(8.22)
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CHAPTER 8. DARK MATTER
is the mean square velocity (weighted by galaxy mass) of all the galaxies in
the cluster.
It is also useful to define the moment of inertia of the cluster as
I≡
X
i
mi |~xi |2 .
(8.23)
The moment of inertia I can be linked to the kinetic energy and the potential
energy if we start by taking the second time derivative of I:
I¨ = 2
X
i
mi (~xi · ~x¨i + ~x˙i · ~x˙i ) .
(8.24)
Using equation (8.20), we can rewrite this as
I¨ = 2
X
i
mi (~xi · ~x¨i ) + 4K .
(8.25)
To introduce the potential energy W into the above relation, we can use
equation (8.17) to write
X
i
mi (~xi · ~x¨i ) = G
X
mi mj
i,j
j6=i
~xi · (~xj − ~xi )
.
|~xj − ~xi |3
(8.26)
However, we could equally well switch around the i and j subscripts to find
the equally valid equation
X
j
mj (~xj · ~x¨j ) = G
Since
X
i
X
mj mi
j,i
i6=j
mi (~xi · ~x¨i ) =
X
j
~xj · (~xi − ~xj )
.
|~xi − ~xj |3
(8.27)
mj (~xj · ~x¨j )
(8.28)
(it doesn’t matter whether we call the variable over which we’re summing i
or j or k or “Fred”), we can combine equations (8.26) and (8.27) to find
X
i


X
1 X
mi (~xi · ~x¨i ) =
mi (~xi · ~x¨i ) +
mj (~xj · ~x¨j )
2 i
j
= −
G X mi mj
=W .
2 i,j |~xj − ~xi |
j6=i
(8.29)
8.3. DARK MATTER IN CLUSTERS
167
Thus, the first term on the right hand side of equation (8.25) is simply 2W ,
and we may now write down the simple relation
I¨ = 2W + 4K .
(8.30)
This relation is known as the virial theorem. It was actually first derived in
the nineteenth century in the context of the kinetic theory of gases, but as
we have seen, it applies perfectly well to a self-gravitating system of point
masses.
The virial theorem is particularly useful when it is applied to a system in
steady state, with a constant moment of inertia. (This implies, among other
things, that the system is neither expanding nor contracting, and that you
are using a coordinate system in which the center of mass of the cluster is at
rest.) If I = constant, then the steady-state virial theorem is
0 = W + 2K ,
(8.31)
K = −W/2 .
(8.32)
or
That is, for a self-gravitating system in steady state, the kinetic energy K
is equal to −1/2 times the potential energy W . Using equation (8.19) and
(8.21) in equation (8.32), we find
1
α GM 2
M hv 2 i =
.
2
2 rh
(8.33)
This means we can use the virial theorem to estimate the mass of a cluster
of galaxies, or any other self-gravitating steady-state system:
M=
hv 2 irh
.
αG
(8.34)
Note the similarity between equation (8.14), used to estimate the mass of a
rotating spiral galaxy, and equation (8.34), used to estimate the mass of a
cluster of galaxies. In either case, we estimate the mass of a self-gravitating
system by multiplying the square of a characteristic velocity by a characteristic radius, then dividing by the gravitational constant G.
Applying the virial theorem to a real cluster of galaxies, such as the Coma
cluster, is complicated by the fact that we have only partial information
about the cluster, and thus do not know hv 2 i and rh exactly. For instance,
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CHAPTER 8. DARK MATTER
we can find the line-of-sight velocity of each galaxy from its redshift, but the
velocity perpendicular to the line of sight is unknown. From measurements of
the redshifts of hundreds of galaxies in the Coma cluster, the mean redshift
of the cluster is found to be
hzi = 0.0232 ,
(8.35)
which can be translated into a radial velocity
hvr i = chzi = 6960 km s−1
(8.36)
dComa = (c/H0 )hzi = 99 Mpc .
(8.37)
and a distance
The velocity dispersion of the cluster along the line of sight is found to be
σr = h(vr − hvr i)2 i1/2 = 880 km s−1 .
(8.38)
If we assume that the velocity dispersion is isotropic, then the three-dimensional
mean square velocity hv 2 i will be equal to three times the one-dimensional
mean square velocity σr2 , yielding
hv 2 i = 3(880 km s−1 )2 = 2.32 × 1012 m2 s−2 .
(8.39)
Estimating the half-mass radius rh of the Coma cluster is even more perilridden than estimating the mean square velocity hv 2 i. After all, we don’t
know the distribution of dark matter in the cluster a priori ; in fact, the
total amount of dark matter is what we’re trying to find out. However,
if we assume that the mass-to-light ratio is constant with radius, then the
sphere containing half the mass of the cluster will be the same as the sphere
containing half the luminosity of the cluster. If we further assume that the
cluster is intrinsically spherical, then the observed distribution of galaxies
within the Coma cluster indicates a half-mass radius
rh ≈ 1.5 Mpc ≈ 4.6 × 1022 m .
(8.40)
After all these assumptions and approximations, we may estimate the mass
of the Coma cluster to be
MComa
(2.32 × 1012 m2 s−2 )(4.6 × 1022 m)
hv 2 irh
≈
=
αG
(0.4)(6.7 × 10−11 m3 s−2 kg−1 )
≈ 4 × 1045 kg ≈ 2 × 1015 M¯ .
(8.41)
(8.42)
169
8.3. DARK MATTER IN CLUSTERS
Thus, less than two percent of the mass of the Coma cluster consists of stars
(MComa,? ≈ 3 × 1013 M¯ ), and only ten percent consists of hot intracluster
gas (MComa,gas ≈ 2 × 1014 M¯ ). Combined with the luminosity of the Coma
cluster, LComa,B = 8 × 1012 L¯,B , the total mass of the Coma cluster implies
a mass-to-light ratio of
hM/LB iComa ≈ 250 M¯ / L¯,B ,
(8.43)
greater than the mass-to-light ratio of our Galaxy.
The presence of a vast reservoir of dark matter in the Coma cluster is
confirmed by the fact that the hot, x-ray emitting intracluster gas, shown
in Figure 8.2, is still in place; if there were no dark matter to anchor the
gas gravitationally, the hot gas would have expanded beyond the cluster
on time scales much shorter than the Hubble time. The temperature and
density of the hot gas in the Coma cluster can be used to make yet another
estimate of the cluster’s mass. If the hot intracluster gas is supported by
its own pressure against gravitational infall, it must obey the equation of
hydrostatic equilibrium:
dP
GM (r)ρ(r)
=−
,
dr
r2
(8.44)
where P is the pressure of the gas, ρ is the density of the gas, and M is the
total mass inside a sphere of radius r, including gas, stars, dark matter, lost
socks, and anything else.6 Of course, the gas in Coma isn’t perfectly spherical
in shape, as equation (8.44) assumes, but it’s close enough to spherical to
give a reasonable approximation to the mass.
The pressure of the gas is given by the perfect gas law,
P =
ρkT
,
µmp
(8.45)
where T is the temperature of the gas, and µ is its mass in units of the
proton mass (mp ). The mass of the cluster, as a function of radius, is found
by combining equations (8.44) and (8.45):
"
d ln ρ d ln T
kT (r)r
−
−
M (r) =
Gµmp
d ln r
d ln r
6
#
.
(8.46)
Equation (8.44) is the same equation which determines the internal structure of a star,
where the inward force due to gravity is also exactly balanced by an outward force due to
a pressure gradient.
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CHAPTER 8. DARK MATTER
The above equation assumes that µ is constant with radius, as you’d expect if the chemical composition and ionization state of the gas is uniform
throughout the cluster.
The x-rays emitted from the hot intracluster gas are a combination of
bremsstrahlung emission (caused by the acceleration of free electrons by protons and helium nuclei) and line emission from highly ionized iron and other
heavy elements. Starting from an x-ray spectrum, it is possible to fit models
to the emission and thus compute the temperature, density, and chemical
composition of the gas. In the Coma cluster, for instance, temperature maps
reveal relatively cool regions (at kT ≈ 5 keV) as well as hotter regions (at
kT ≈ 12 keV), averaging to kT ≈ 9 keV over the entire cluster. The mass
of the Coma cluster, assuming hydrostatic equilibrium, is computed to be
(3 → 4)×1014 M¯ within 0.7 Mpc of the cluster center and (1 → 2)×1015 M¯
within 3.6 Mpc of the center, consistent with the mass estimate of the virial
theorem.
Other clusters of galaxies besides the Coma cluster have had their masses
estimated, using the virial theorem applied to their galaxies or the equation
of hydrostatic equilibrium applied to their gas. Typical mass-to-light ratios
for clusters lie in the range hM/LB i = 200 → 300 M¯ / L¯,B , so the Coma
cluster is not unusual in the amount of dark matter which it contains. If the
masses of all the clusters of galaxies are added together, it is found that their
density parameter is
Ωclus,0 ≈ 0.2 .
(8.47)
This provides a lower limit to the matter density of the universe, since any
matter which is smoothly distributed in the intercluster voids will not be
included in this number.
8.4
Gravitational lensing
So far, I have outlined the classical methods for detecting dark matter via its
gravitational effects on luminous matter.7 We can detect dark matter around
spiral galaxies because it affects the motions of stars and interstellar gas. We
can detect dark matter in clusters of galaxies because it affects the motions of
7
The roots of these methods can be traced back as far as the year 1846, when Leverrier
and Adams deduced the existence of the dim planet Neptune by its effect on the orbit of
Uranus.
171
8.4. GRAVITATIONAL LENSING
Figure 8.5: Deflection of light by a massive compact object.
galaxies and intracluster gas. However, as Einstein realized, dark matter will
affect not only the trajectory of matter, but also the trajectory of photons.
Thus, dark matter can bend and focus light, acting as a gravitational lens.
The effects of dark matter on photons have been used to search for dark
matter within the halo of our own Galaxy, as well as in distant clusters of
galaxies.
To see how gravitational lensing can be used to detect dark matter, start
by considering the dark halo surrounding our Galaxy. Some of the dark
matter in the halo might consist of massive compact objects such as brown
dwarfs, white dwarfs, neutron stars, and black holes. These objects have
been collectively called MACHOs, a slightly strained acronym for MAssive
Compact Halo Objects. If a photon passes such a compact massive object
at an impact parameter b, as shown in Figure 8.5, the local curvature of
space-time will cause the photon to be deflected by an angle
α=
4GM
,
c2 b
(8.48)
where M is the mass of the compact object. For instance, light from a distant
star which just grazes the Sun’s surface should be deflected through an angle
α=
4G M¯
= 1.7 arcsec .
c2 R¯
(8.49)
In 1919, after Einstein predicted a deflection of this magnitude, an eclipse
expedition photographed stars in the vicinity of the Sun. Comparison of the
eclipse photographs with photographs of the same star field taken six months
earlier revealed that the apparent positions of the stars were deflected by the
amount which Einstein had predicted. This result brought fame to Einstein
and experimental support to the theory of general relativity.
Since a star, or a brown dwarf, or a stellar remnant, can deflect light,
it can act as a lens. Suppose a MACHO in the halo of our Galaxy passes
172
CHAPTER 8. DARK MATTER
Figure 8.6: Light from star in the Large Magellanic Cloud is deflected by a
MACHO on its way to an observer in the disk of our Galaxy (seen edge-on
in this figure).
directly between an observer in our Galaxy and a star in the Large Magellanic
Cloud. Figure 8.6 shows such a situation, with a MACHO which happens to
be halfway between the observer and the star. As the MACHO deflects the
light from the distant star, it produces an image of the star which is both
distorted and amplified. If the MACHO is exactly along the line of sight
between the observer and the lensed star, the image produced is a perfect
ring, with angular radius
θE =
µ
4GM 1 − x
c2 d
x
¶1/2
,
(8.50)
where M is the mass of the lensing MACHO, d is the distance from the
observer to the lensed star, and xd (where 0 < x < 1) is the distance from
the observer to the lensing MACHO. The angle θE is known as the Einstein
radius. If x ≈ 0.5 (that is, if the MACHO is roughly halfway between the
observer and the lensed star), then
Ã
M
θE ≈ 4 × 10 arcsec
1 M¯
−4
!1/2 Ã
d
50 kpc
!−1/2
.
(8.51)
If the MACHO does not lie perfectly along the line of sight to the star, then
the image of the star will be distorted into two or more arcs instead of a single
unbroken ring. Although the Einstein radius for an LMC star being lensed
by a MACHO is too small to be resolved, it is possible, in some cases, to
173
8.4. GRAVITATIONAL LENSING
detect the amplification of the flux from the star. For the amplification to be
significant, the angular distance between the MACHO and the lensed star, as
seen from Earth, must be comparable to, or smaller than, the Einstein radius.
Given the small size of the Einstein radius, the probability of any particular
star in the LMC being lensed at any moment is tiny. It has been calculated
that if the dark halo of our galaxy were entirely composed of MACHOs, then
the probability of any given star in the LMC being lensed at any given time
would still only be P ∼ 5 × 10−7 .
To detect lensing by MACHOs, various research groups took up the
daunting task of monitoring millions of stars in the Large Magellanic Cloud
to watch for changes in their flux. Since the MACHOs in our dark halo and
the stars in the LMC are in constant relative motion, the typical signature
of a “lensing event” is a star which becomes brighter as the angular distance
between star and MACHO decreases, then becomes dimmer as the angular
distance increases again. The typical time scale for a lensing event is the
time it takes a MACHO to travel through an angular distance equal to θE as
seen from Earth; for a MACHO halfway between here and the LMC, this is
Ã
d θE
M
∆t =
≈ 90 days
2v
1 M¯
!1/2 µ
v
200 km s−1
¶−1
,
(8.52)
where v is the relative transverse velocity of the MACHO and the lensed
star as seen by the observer on Earth. Generally speaking, more massive
MACHOs produce larger Einstein rings and thus will amplify the lensed star
for a longer time.
The research groups which searched for MACHOs found a scarcity of short
duration lensing events, suggesting that there is not a significant population
of brown dwarfs (with M < 0.08 M¯ ) in the dark halo of our Galaxy. The
total number of lensing events which they detected suggest that as much as
20% of the halo mass could be in the form of MACHOs. The long time scales
of the observed lensing events, which have ∆t > 35 days, suggest typical
MACHO masses of M > 0.15 M¯ . (Perhaps the MACHOs are old, cold white
dwarfs, which would have the correct mass.) Alternatively, the observed
lensing events could be due, at least in part, to lensing objects within the
LMC itself. In any case, the search for MACHOs suggests that most of
the matter in the dark halo of our galaxy is due to a smoothly distributed
component, instead of being congealed into MACHOs of roughly stellar mass.
Gravitational lensing occurs at all mass scales. Suppose, for instance,
that a cluster of galaxies, with M ∼ 1014 M¯ , at a distance ∼ 500 Mpc from
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CHAPTER 8. DARK MATTER
Figure 8.7: A Hubble Space Telescope picture of the rich cluster Abell 2218,
displaying gravitationally lensed arcs. The region shown is roughly 2.4 arcmin
by 1.2 arcmin, equivalent to 0.54 Mpc by 0.27 Mpc at the distance of Abell
2218 (courtesy of W. Couch [University of New South Wales] and NASA).
our Galaxy, lenses a background galaxy at d ∼ 1000 Mpc. The Einstein
radius for this configuration will be
Ã
M
θE ≈ 0.5 arcmin
14
10 M¯
!1/2 Ã
d
1000 Mpc
!−1/2
.
(8.53)
The arc-shaped images into which the background galaxy is distorted by
the lensing cluster can thus be resolved. For instance, Figure 8.7 shows
an image of the cluster Abell 2218, which has a redshift z = 0.18, and
hence is at a proper distance d = 770 Mpc. The elongated arcs seen in
Figure 8.7 are not oddly shaped galaxies within the cluster; instead, they
are background galaxies, at redshifts z > 0.18, which are gravitationally
lensed by the cluster mass. The mass of clusters can be estimated by the
degree to which they lens background galaxies. The masses calculated in this
way are in general agreement with the masses found by applying the virial
theorem to the motions of galaxies in the cluster or by applying the equation
of hydrostatic equilibrium to the hot intracluster gas.
8.5. WHAT’S THE MATTER?
8.5
175
What’s the matter?
I have described how to detect dark matter by its gravitational effects, but
I’ve been dodging the essential question: “What is it? ” Adding together
the masses of clusters of galaxies gives a lower limit on the matter density
of the universe, telling us that Ωm,0 ≥ 0.2. However, the density parameter
of baryonic matter is only Ωbary,0 ≈ 0.04. Thus, the density of nonbaryonic
matter is at least four times the density of the familiar baryonic matter of
which people and planets and stars are made.
As you might expect, conjecture about the nature of the nonbaryonic
dark matter has run rampant (some might even say it has run amok). A
component of the universe which is totally invisible is an open invitation to
speculation. To give a taste of the variety of speculation, some scientists
have proposed that the dark matter might be made of axions, a type of
elementary particle with a rest energy of max c2 ∼ 10−5 eV, equivalent to
max ∼ 2 × 10−41 kg. This is a rather low mass – it would take some 50 billion
axions (if they indeed exist) to equal the mass of one electron. On the other
hand, some scientists have conjectured that the dark matter might be made
of primordial black holes, with masses up to mBH ∼ 105 M¯ , equivalent to
mBH ∼ 2 × 1035 kg.8 This is a rather high mass – it would take some 30
billion Earths to equal the mass of one primordial black hole (if they indeed
exist). It is a sign of the vast ignorance concerning nonbaryonic dark matter
that these two candidates for the role of dark matter differ in mass by 76
orders of magnitude.
One nonbaryonic particle which we know exists, and which seems to have
a non-zero mass, is the neutrino. As stated in section 5.1, there should exist
today a cosmic background of neutrinos. Just as the Cosmic Microwave
Background is a relic of the time when the universe was opaque to photons,
the Cosmic Neutrino Background is a relic of the time when the universe
was hot and dense enough to be opaque to neutrinos. The number density of
each of the three flavors of neutrinos (νe , νµ , and ντ ) has been calculated to
be 3/11 times the number density of CMB photons, yielding a total number
density of neutrinos
nν = 3(3/11)nγ = (9/11)(4.11 × 108 m−3 ) = 3.36 × 108 m−3 .
8
(8.54)
A primordial black hole is one which forms very early in the history of the universe,
rather than by the collapse of a massive star later on.
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CHAPTER 8. DARK MATTER
This means that at any moment, about twenty million cosmic neutrinos are
zipping through your body, “like photons through a pane of glass”. In order
to provide all the nonbaryonic mass in the universe, the average neutrino
mass would have to be
Ωdm,0 εc,0
.
(8.55)
mν c2 =
nν
Given a density parameter in nonbaryonic dark matter of Ωdm,0 ≈ 0.26, this
implies that a mean neutrino mass of
mν c2 ≈
0.26(5200 MeV m−3 )
≈ 4 eV
3.36 × 108 m−3
(8.56)
would be necessary to provide all the nonbaryonic dark matter in the universe.
Evidence indicates that neutrinos do have some mass. But how much?
Enough to contribute significantly to the energy density of the universe?
The observations of neutrinos from the Sun, as mentioned in section 2.4,
indicate that electron neutrinos oscillate into some other flavor of neutrino,
with the difference in the squares of the masses of the two neutrinos being
∆(m2ν c4 ) ≈ 3 × 10−5 eV2 . Observations of muon neutrinos created in the
Earth’s atmosphere indicate that muon neutrinos oscillate into tau neutrinos,
with ∆(m2ν c4 ) ≈ 3 × 10−3 eV2 . The minimum neutrino masses consistent
with these results would have one flavor with mν c2 ∼ 0.05 eV, another with
mν c2 ∼ 0.005 eV, and the third with mν c2 ¿ 0.005 eV. If the neutrino masses
are this small, then the density parameter in neutrinos is only Ων ∼ 10−3 , and
neutrinos make up less than 0.5% of the nonbaryonic dark matter. If, on the
other hand, neutrinos make up all the nonbaryonic dark matter, the masses
of the three species would have to be very nearly identical; for instance, one
neutrino flavor with mν c2 = 4.0 eV, another with mν c2 = 4.0004 eV, and
the third with mν c2 = 4.000004 eV would be in agreement with the deduced
values of ∆(m2ν c4 ).
If the masses of all three neutrinos turn out to be significantly less than
mν c2 ∼ 4 eV, then the bulk of the nonbaryonic dark matter in the universe
must be made of some particle other than neutrinos. Particle physicists have
provided several possible candidates for the role of dark matter. For instance,
consider the extension of the Standard Model of particle physics known as
supersymmetry. Various supersymmetric models predict the existence of
massive nonbaryonic particles such as photinos, gravitinos, axinos, sneutrinos, gluinos, and so forth. None of these “inos” have yet been detected in
8.5. WHAT’S THE MATTER?
177
laboratories. The fact that supersymmetric particles such as photinos have
not yet been seen in particle accelerator experiments means that they must
be massive (if they exist), with mc2 > 10 GeV.
Like neutrinos, the hypothetical supersymmetric particles interact with
other particles only through gravity and through the weak nuclear force,
which makes them intrinsically difficult to detect. Particles which interact
via the weak nuclear force, but which are much more massive than the upper
limit on the neutrino mass, are known generically as Weakly Interacting
Massive Particles, or WIMPs.9 Since WIMPs, like neutrinos, do interact
with atomic nuclei on occasion, experimenters have set up WIMP detectors
to discover cosmic WIMPs. So far, no convincing detections have been made
– but the search goes on.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Liddle (1999), ch. 8: A brief sketch of methods for detecting dark matter.
Peacock (1999), ch. 12: Dark matter in the universe, both baryonic and
nonbaryonic. Also, chapter 4 gives a good review of gravitational lensing.
Rich (2001), ch. 2.4: A discussion of the dark matter candidates.
Problems
(8.1) Suppose it were suggested that black holes of mass 10−8 M¯ made up
all the dark matter in the halo of our Galaxy. How far away would
you expect the nearest such black hole to be? How frequently would
you expect such a black hole to pass within 1 AU of the Sun? (An
order-of-magnitude estimate is sufficient.)
Suppose it were suggested that MACHOs of mass 10−3 M¯ (about the
mass of Jupiter) made up all the dark matter in the halo of our Galaxy.
How far away would you expect the nearest MACHO to be? How
9
The acronym “MACHO”, encountered in the previous section, was first coined as a
humorous riposte to the acronym “WIMP”.
178
CHAPTER 8. DARK MATTER
frequently would such a MACHO pass within 1 AU of the Sun? (Again,
an order-of-magnitude estimate will suffice.)
(8.2) The Draco galaxy is a dwarf galaxy within the Local Group. Its luminosity is L = (1.8 ± 0.8) × 105 L¯ and half its total luminosity is
contained within a sphere of radius rh = 120 ± 12 pc. The red giant
stars in the Draco galaxy are bright enough to have their line-of-sight
velocities measured. The measured velocity dispersion of the red giant
stars in the Draco galaxy is σr = 10.5 ± 2.2 km s−1 . What is the mass
of the Draco galaxy? What is its mass-to-light ratio? Describe the
possible sources of error in your mass estimate of this galaxy.
(8.3) A light ray just grazes the surface of the Earth (M = 6.0 × 1024 kg,
R = 6.4 × 106 m). Through what angle α is the light ray bent by gravitational lensing? (Ignore the refractive effects of the Earth’s atmosphere.) Repeat your calculation for a white dwarf (M = 2.0 × 1030 kg,
R = 1.5 × 107 m) and for a neutron star (M = 3.0 × 1030 kg, R =
1.2 × 104 m).
(8.4) If the halo of our Galaxy is spherically symmetric, what is the mass
density ρ(r) within the halo? If the universe contains a cosmological
constant with density parameter ΩΛ,0 = 0.7, would you expect it to
significantly affect the dynamics of our Galaxy’s halo? Explain why or
why not.
(8.5) In the previous chapter, I noted that galaxies in rich clusters are poor
standard candles, because they tend to grow brighter with time as
they merge with other galaxies. Let’s estimate the galaxy merger rate
in the Coma cluster to see whether it’s truly significant. The Coma
cluster contains N ≈ 1000 galaxies within its half-mass radius of rh ≈
1.5 Mpc. What is the mean number density of galaxies within the
half-mass radius? Suppose that the typical cross-section of a galaxy is
Σ ≈ 10−3 Mpc2 . How far will a galaxy in the Coma cluster travel, on
average, before it collides with another galaxy? The velocity dispersion
of the Coma cluster is σ ≈ 880 km s−3 . What is the average time
between collisions for a galaxy in the Coma cluster? Is this time greater
than or less than the Hubble time?
Chapter 9
The Cosmic Microwave
Background
If Heinrich Olbers had lived in intergalactic space and had eyes that operated
at millimeter wavelengths (admittedly a very large “if”), he would not have
formulated Olbers’ Paradox. At wavelengths of a few millimeters, thousands
of times longer than human eyes can detect, most of the light in the universe
comes not from the hot balls of gas we call “stars”, but from the Cosmic
Microwave Background (CMB). The night sky, unknown to Olbers, actually
is uniformly bright – it’s just uniformly bright at a temperature of T 0 =
2.725 K rather than at a temperature of a few thousand degrees Kelvin. The
current energy density of the Cosmic Microwave Background,
εγ,0 = αT04 = 0.261 MeV m−3 ,
(9.1)
is only 5 × 10−5 times the current critical density. However, since the energy
per CMB photon is small (hfmean = 6.34 × 10−4 eV), the number density of
CMB photons in the universe is large:
nγ,0 = 4.11 × 108 m−3 .
(9.2)
It is particularly enlightening to compare the energy density and number
density of photons to those of baryons (that is, protons and neutrons). Given
a current density parameter for baryons of Ωbary,0 ≈ 0.04, the current energy
density of baryons is
εbary,0 = Ωbary,0 εc,0 ≈ 0.04(5200 MeV m−3 ) ≈ 210 MeV m−3 .
179
(9.3)
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Thus, the energy density in baryons today is about 800 times the energy
density in CMB photons. Note, though, that the rest energy of a proton
or neutron is Ebary ≈ 939 MeV; this is more than a trillion times the mean
energy of a CMB photon. The number density of baryons, therefore, is much
lower than the number density of photons:
nbary,0 =
εbary,0
210 MeV m−3
≈
≈ 0.22 m−3 .
Ebary
939 MeV
(9.4)
The ratio of baryons to photons in the universe (a number usually designated
by the Greek letter η) is, from equations (9.2) and (9.4),
η=
nbary,0
0.22 m−3
≈ 5 × 10−10 .
≈
nγ,0
4.11 × 108 m−3
(9.5)
Baryons are badly outnumbered by photons in the universe as a whole, by a
ratio of roughly two billion to one.
9.1
Observing the CMB
Although CMB photons are as common as dirt,1 Arno Penzias and Robert
Wilson were surprised when they serendipitously discovered the Cosmic Microwave Background. At the time of their discovery, Penzias and Wilson were
radio astronomers working at Bell Laboratories. The horn-reflector radio antenna which they used had previously been utilized to receive microwave
signals, of wavelength λ = 7.35 cm, reflected from an orbiting communications satellite. Turning from telecommunications to astronomy, Penzias and
Wilson found a slightly stronger signal than they expected when they turned
the antenna toward the sky. They did everything they could think of to reduce “noise” in their system. They even shooed away a pair of pigeons that
had roosted in the antenna and cleaned up they later called “the usual white
dielectric” generated by pigeons.
The excess signal remained. It was isotropic and constant with time, so it
couldn’t be associated with an isolated celestial source. Wilson and Penzias
were puzzled until they were put in touch with Robert Dicke and his research
group at Princeton University. Dicke had deduced that the universe, if it
1
Actually, much commoner than dirt, when you stop to think of it, since dirt is made
of baryons.
9.1. OBSERVING THE CMB
181
started in a hot dense state, should now be filled with microwave radiation.2
In fact, Dicke and his group were in the process of building a microwave
antenna when Penzias and Wilson told them that they had already detected
the predicted microwave radiation. Penzias and Wilson wrote a paper for The
Astrophysical Journal in which they wrote, “Measurements of the effective
zenith noise temperature of the 20-foot horn-reflector antenna . . . at 4080
Mc/s have yielded a value about 3.5 K higher than expected. This excess
temperature is, within the limits of our observations, isotropic, unpolarized,
and free from seasonal variations (July, 1964 – April, 1965). A possible
explanation for the observed excess noise temperature is the one given by
Dicke, Peebles, Roll, and Wilkinson in a companion letter in this issue.” The
companion paper by Dicke and his collaborators points out that the radiation
could be a relic of an early, hot, dense, and opaque state of the universe.
Measuring the spectrum of the CMB, and confirming that it is indeed a
blackbody, is not a simple task, even with modern technology. The current
energy per CMB photon, ∼ 6 × 10−4 eV, is tiny compared to the energy
required to break up an atomic nucleus (∼ 1 MeV) or even the energy required
to ionize an atom (∼ 10 eV). However, the mean photon energy is comparable
to the energy of vibration or rotation for a small molecule such as H2 O.
Thus, CMB photons can zip along for more than 13 billion years through the
tenuous intergalactic medium, then be absorbed a microsecond away from
the Earth’s surface by a water molecule in the atmosphere. Microwaves with
wavelengths shorter than λ ∼ 3 cm are strongly absorbed by water molecules.
Penzias and Wilson observed the CMB at a wavelength λ = 7.35 cm because
that was the wavelength of the signals that Bell Labs had been bouncing off
orbiting satellites. Thus, Penzias and Wilson were observing at a wavelength
40 times longer than the wavelength (λ ≈ 2 mm) at which the CMB spectrum
reaches its peak.
The CMB can be measured at wavelengths shorter than 3 cm by observing from high-altitude balloons or from the South Pole, where the combination of cold temperatures and high altitude3 keeps the atmospheric humidity
low. The best way to measure the spectrum of the CMB, however, is to go
completely above the damp atmosphere of the Earth. The CMB spectrum
2
To give credit where it’s due, the existence of the cosmic background radiation had
actually been predicted by the physicist George Gamow and his colleagues as early as
1948; unfortunately, Gamow’s prediction wasn’t acted on at the time he made it, and had
fallen into obscurity during the intervening years.
3
The South Pole is nearly 3 kilometers above sea level.
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Figure 9.1: The spectrum of the Cosmic Microwave Background, as measured
by the FIRAS instrument on the COBE satellite. The uncertainties in the
measurement are smaller than the thickness of the line (from Fixsen et al.
1996, ApJ, 473, 576).
was first measured accurately over a wide range of wavelengths by the COsmic Background Explorer (COBE) satellite, launched in 1989, into an orbit
900 km above the Earth’s surface. COBE actually contained three different
instruments. The Diffuse InfraRed Background Experiment (DIRBE) was
designed to measure radiation at the wavelengths 0.001 mm < λ < 0.24 mm;
at these wavelengths, it was primarily detecting stars and dust within our
own Galaxy. The second instrument, called the Far InfraRed Absolute Spectrophotometer (FIRAS), was used to measure the spectrum of the CMB in
the range 0.1mm < λ < 10 mm, a wavelength band which includes the peak in
the CMB spectrum. The third instrument, called the Differential Microwave
Radiometer (DMR), was designed to make full-sky maps of the CMB at three
different wavelengths: λ = 3.3 mm, 5.7 mm, and 9.6 mm. Three important
results came from the analysis of the COBE data.
Result number one: At any angular position (θ, φ) on the sky, the spectrum of the Cosmic Microwave Background is very close to that of an ideal
blackbody, as illustrated in Figure 9.1. How close is very close? FIRAS could
have detected fluctuations in the spectrum as small as ∆²/² ≈ 10−4 . No deviations were found at this level within the wavelength range investigated by
9.1. OBSERVING THE CMB
183
FIRAS.
Result number two: The CMB has the dipole distortion in temperature
shown in the top panel of Figure 9.2.4 That is, although each point on the
sky has a blackbody spectrum, in one half of the sky the spectrum is slightly
blueshifted to higher temperatures, and in the other half the spectrum is
slightly redshifted to lower temperatures.5 This dipole distortion is a simple
Doppler shift, caused by the net motion of the COBE satellite relative to a
frame of reference in which the CMB is isotropic. After correcting for the
orbital motion of COBE around the Earth (v ∼ 8 km s−1 ), for the orbital
motion of the Earth around the Sun (v ∼ 30 km s−1 ), for the orbital motion
of the Sun around the Galactic center (v ∼ 220 km s−1 ), and for the orbital
motion of our Galaxy relative to the center of mass of the Local Group
(v ∼ 80 km s−1 ), it is found that the Local Group is moving in the general
direction of the constellation Hydra, with a speed vLG = 630 ± 20 km s−1 =
0.0021c. This peculiar velocity for the Local Group is what you’d expect as
the result of gravitational acceleration by the largest lumps of matter in the
vicinity of the Local Group. The Local Group is being accelerated toward
the Virgo cluster, the nearest big cluster to us. In addition, the Virgo cluster
is being accelerated toward the Hydra-Centaurus supercluster, the nearest
supercluster to us. The combination of these two accelerations, working over
the age of the universe, has launched the Local Group in the direction of
Hydra, at 0.2% of the speed of light.
Result number three: After the dipole distortion of the CMB is subtracted
away, the remaining temperature fluctuations, shown in the lower panel of
Figure 9.2, are small in amplitude. Let the temperature of the CMB, at a
given point on the sky, be T (θ, φ). The mean temperature, averaging over
all locations, is
1 Z
hT i =
T (θ, φ) sin θ dθ dφ = 2.725 K .
(9.6)
4π
The dimensionless temperature fluctuation at a given point on the sky is
δT
T (θ, φ) − hT i
(θ, φ) ≡
.
(9.7)
T
hT i
4
The dipole distortion of the CMB was first detected in 1977, using aircraft-borne and
balloon-borne detectors. The unique contribution of COBE was the precision with which
it measured the temperature distortion.
5
The distorted “yin-yang” pattern in the upper panel of Figure 9.2 represents the
darker, cooler (yin?) hemisphere of the sky and the hotter, brighter (yang?) hemisphere,
distorted by the map projection.
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Figure 9.2: Top panel: The fluctuations in temperature in the CMB, as measured by COBE. Bottom panel: The fluctuations in temperature remaining
after subtraction of the dipole due to the satellite’s proper motion. The band
across the middle is due to emission from the disk of our own Galaxy. (Courtesy of NASA Goddard Space Flight Center and the COBE Science Working
Group.)
9.2. RECOMBINATION AND DECOUPLING
185
From the maps of the sky made by the DMR instrument aboard COBE, it
was found that after subtraction of the Doppler dipole, the root mean square
temperature fluctuation was
*Ã
δT
T
!2 +1/2
= 1.1 × 10−5 .
(9.8)
(This analysis excludes the regions of the sky contaminated by foreground
emission from our own Galaxy.) The fact that the temperature of the CMB
varies by only 30 microKelvin across the sky represents a remarkably close
approach to isotropy.6
The observations that the CMB has a nearly perfect blackbody spectrum
and that it is nearly isotropic (once the Doppler dipole is removed) provide
strong support for the Hot Big Bang model of the universe. A background of
nearly isotropic blackbody radiation is natural if the universe was once hot,
dense, opaque, and nearly homogeneous, as it was in the Hot Big Bang scenario. If the universe did not go through such a phase, then any explanation
of the Cosmic Microwave Background will have to be much more contrived.
9.2
Recombination and decoupling
To understand in more detail the origin of the Cosmic Microwave Background, we’ll have to examine fairly carefully the process by which the baryonic matter goes from being an ionized plasma to a gas of neutral atoms, and
the closely related process by which the universe goes from being opaque to
being transparent. To avoid muddle, I will distinguish among three closely
related (but not identical) moments in the history of the universe. First,
the epoch of recombination is the time at which the baryonic component
of the universe goes from being ionized to being neutral. Numerically, you
might define it as the instant in time at which the number density of ions is
equal to the number density of neutral atoms.7 Second, the epoch of photon
decoupling is the time at which the rate at which photons scatter from electrons becomes smaller than the Hubble parameter (which tells us the rate at
6
To make an analogy, if the surface of the Earth were smooth to 11 parts per million,
the highest mountains would be just seventy meters above the deepest ocean trenches.
7
Cosmologists sometimes grumble that this should really be called the epoch of “combination” rather than the epoch of “recombination”, since this is the very first time when
electrons and ions combined to form neutral atoms.
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Figure 9.3: An observer is surrounded by a spherical last scattering surface.
The photons of the CMB travel straight to us from the last scattering surface,
being continuously redshifted.
which the universe expands). When photons decouple, they cease to interact
with the electrons, and the universe becomes transparent. Third, the epoch
of last scattering is the time at which a typical CMB photon underwent its
last scattering from an electron. Surrounding every observer in the universe
is a last scattering surface, illustrated in Figure 9.3, from which the CMB
photons have been streaming freely, with no further scattering by electrons.
The probability that a photon will scatter from an electron is small once the
expansion rate of the universe is faster than the scattering rate; thus, the
epoch of last scattering is very close to the epoch of photon decoupling.
To keep things from getting too complicated, I will assume that the baryonic component of the universe consisted entirely of hydrogen at the epoch
of recombination. This is not, I concede, a strictly accurate assumption.
Even at the time of recombination, before stars had a chance to pollute
the universe with heavy elements, there was a significant amount of helium
present.8 However, the presence of helium is merely a complicating factor.
All the significant physics of recombination can be studied in a simplified
8
In the next chapter, we will examine how and why this helium was formed in the early
universe.
9.2. RECOMBINATION AND DECOUPLING
187
universe containing no elements other than hydrogen. The hydrogen can
take the form of a neutral atom (designated by the letter H), or of a naked
hydrogen nucleus, otherwise known as a proton (designated by the letter p).
To maintain charge neutrality in this hydrogen-only universe, the number
density of free electrons must be equal to that of free protons: ne = np .
The degree to which the baryonic content of the universe is ionized can be
expressed as the fractional ionization X, defined as
X≡
np
ne
np
=
=
.
np + n H
nbary
nbary
(9.9)
The value of X ranges from X = 1 when the baryonic content is fully ionized
to X = 0 when it consists entirely of neutral atoms.
One useful consequence of assuming that hydrogen is the only element is
that there is now a single relevant energy scale in the problem: the ionization
energy of hydrogen, Q = 13.6 eV. A photon with an energy hf > Q is capable
of photoionizing a hydrogen atom:
H + γ → p + e− .
(9.10)
This reaction can run in the opposite direction, as well; a proton and an
electron can undergo radiative recombination, forming a bound hydrogen
atom while a photon carries away the excess energy:
p + e− → H + γ .
(9.11)
In a universe containing protons, electrons, and photons, the fractional ionization X will depend on the balance between photoionization and radiative
recombination.
Let’s travel back in time to a period before the epoch of recombination.
For concreteness, let’s choose the moment when a = 10−5 , corresponding
to a redshift z = 105 . (In the Benchmark Model, this scale factor was
reached when the universe was seventy years old.) The temperature of the
background radiation at this time was was T ≈ 3 × 105 K, and the average
photon energy was hfmean ≈ 2.7kT ≈ 60 eV. With such a high energy per
photon, and with a ratio of photons to baryons of nearly two billion, any hydrogen atoms that happened to form by radiative recombination were very
short-lived; almost immediately, they were blasted apart into their component electron and proton by a high-energy photon. At early times, then, the
fractional ionization of the universe was very close to X = 1.
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
When the universe was fully ionized, photons interacted primarily with
electrons, and the main interaction mechanism was Thomson scattering:
γ + e− → γ + e − .
(9.12)
The scattering interaction is accompanied by a transfer of energy and momentum between the photon and electron. The cross-section for Thomson
scattering is the Thomson cross-section of σe = 6.65 × 10−29 m2 . The mean
free path of a photon – that is, the mean distance it travels before scattering
from an electron – is
1
λ=
.
(9.13)
ne σ e
Since photons travel with a speed c, the rate at which a photon undergoes
scattering interactions is
c
(9.14)
Γ = = n e σe c .
λ
When the baryonic component of the universe is fully ionized, ne = np =
nbary . Currently, the number density of baryons is nbary,0 = 0.22 m−3 . The
number density of conserved particles, such as baryons, goes as 1/a3 , so when
the early universe was fully ionized, the free electron density was
ne = nbary =
nbary,0
,
a3
(9.15)
and the scattering rate for photons was
Γ=
4.4 × 10−21 s−1
nbary,0 σe c
=
.
a3
a3
(9.16)
This means, for instance, that at a = 10−5 , photons would scatter from
electrons at a rate Γ = 4.4 × 10−6 s−1 , about three times a week.
The photons remain coupled to the electrons as long as their scattering
rate, Γ, is larger than H, the rate at which the universe expands; this is
equivalent to saying that their mean free path λ is shorter than the Hubble distance c/H. As long as photons scatter frequently from electrons, the
photons remain in thermal equilibrium with the electrons (and, indirectly,
with the protons as well, thanks to the electrons’ interactions with the protons). The photons, electrons, and protons, as long as they remain in thermal
equilibrium, all have the same temperature T . When the photon scattering
rate Γ drops below H, then the electrons are being diluted by expansion
9.3. THE PHYSICS OF RECOMBINATION
189
more rapidly than the photons can interact with them. The photons then
decouple from the electrons and the universe becomes transparent. Once
the photons are decoupled from the electrons and protons, the baryonic portion of the universe is no longer compelled to have the same temperature
as the Cosmic Microwave Background. During the early stages of the universe (a < arm ≈ 3 × 10−4 ) the universe was radiation dominated, and the
Friedmann equation was
Ωr,0
H2
= 4 .
(9.17)
2
H0
a
Thus, the Hubble parameter was
1/2
H0 Ωr,0
2.1 × 10−20 s−1
=
,
H=
a2
a2
(9.18)
This means, for instance, that at a = 10−5 , the Hubble parameter was H =
2.1 × 10−10 s−1 . Since this is much smaller than the scattering rate Γ =
4.4 × 10−6 s−1 at the same scale factor, the photons were well coupled to the
electrons and protons.
If hydrogen remained ionized (and note the qualifying if ), then photons
would have remained coupled to the electrons and protons until a relatively
recent time. Taking into account the transition from a radiation-dominated
to a matter-dominated universe, and the resulting change in the expansion
rate, we can compute that if hydrogen had remained fully ionized, then
decoupling would have taken place at a scale factor a ≈ 0.023, corresponding
to a redshift z ≈ 42 and a CMB temperature of T ≈ 120 K. However, at
such a low temperature, the CMB photons are too low in energy to keep the
hydrogen ionized. Thus, the decoupling of photons is not a gradual process,
caused by the continuous lowering of free electron density as the universe
expands. Rather, it is a relatively sudden process, caused by the abrupt
plummeting of free electron density during the epoch of recombination, as
electrons combine with protons to form hydrogen atoms.
9.3
The physics of recombination
When does recombination, and the consequent photon decoupling, take place?
It’s easy to do a quick and dirty approximation of the recombination temperature. Recombination, one could argue, must take place when the mean
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
energy per photon of the Cosmic Microwave Background falls below the ionization energy of hydrogen, Q = 13.6 eV. When this happens, the average
CMB photon is no longer able to photoionize hydrogen. Since the mean
CMB photon energy is ∼ 2.7kT , this line of argument would indicate a recombination temperature of
Trec ∼
13.6 eV
Q
∼
∼ 60,000 K .
2.7k
2.7(8.6 × 10−5 eV K−1 )
(9.19)
Alas, this crude approximation is a little too crude to be useful. It doesn’t
take into account the fact that CMB photons are not of uniform energy – a
blackbody spectrum has an exponential tail (see Figure 2.7) trailing off to
high energies. Although the mean photon energy is 2.7kT , about one photon
in 500 will have E > 10kT , one in 3 million will have E > 20kT , and one
in 30 billion will have E > 30kT . Although extremely high energy photons
make up only a tiny fraction of the CMB photons, the total number of CMB
photons is enormous – nearly 2 billion photons for every baryon. The vast
swarms of photons that surround every newly formed hydrogen atom greatly
increase the probability that the atom will collide with a photon from the
high-energy tail of the blackbody spectrum, and be photoionized.
Thus, we expect the recombination temperature to depend on the baryonto-photon ratio η as well as on the ionization energy Q. An exact calculation
of the fractional ionization X, as a function of η and T , requires a smattering
of statistical mechanics. Let’s start with the reaction that determines the
value of X in the early universe:
H+γ *
) p + e− .
(9.20)
While the photons are still coupled to the baryonic component, this reaction
will be in statistical equilibrium, with the photoionization rate (going from
left to right) balancing the radiative recombination rate (going from right
to left). When a reaction is in statistical equilibrium at a temperature T ,
the number density nx of particles with mass mx is given by the MaxwellBoltzmann equation
nx = g x
Ã
mx kT
2πh̄2
!3/2
mx c2
exp −
kT
Ã
!
,
(9.21)
as long as the particles are non-relativistic, with kT ¿ mx c2 . In equation (9.21), gx is the statistical weight of particle x. For instance, electrons,
191
9.3. THE PHYSICS OF RECOMBINATION
protons, and neutrons (and their anti-particles as well) all have a statistical
weight gx = 2, corresponding to their two possible spin states.9 From the
Maxwell-Boltzmann equation for H, p, and e− , we can construct an equation
which relates the number densities of these particles:
nH
gH
=
np ne
gp g e
Ã
mH
mp me
!3/2 Ã
kT
2πh̄2
!−3/2
[mp + me − mH ]c2
exp
kT
Ã
!
.
(9.22)
Equation (9.22) can be simplified further. First, since the mass of an electron
is small compared to that of a photon, we can set mH /mp = 1. Second, the
binding energy Q = 13.6 eV is given by the formula (mp + me − mH )c2 = Q.
The statistical weights of the proton and electron, are gp = ge = 2, while the
statistical weight of a hydrogen atom is gH = 4. Thus, the factor gH /(gp ge )
can be set equal to one. The resulting equation,
nH
=
np ne
Ã
me kT
2πh̄2
!−3/2
exp
µ
Q
kT
¶
,
(9.23)
is called the Saha equation. Our next job is to convert the Saha equation
into a relation among X, T , and η. From the definition of X (equation 9.9),
we can make the substitution
nH =
1−X
np ,
X
(9.24)
and from the requirement of charge neutrality, we can make the substitution
ne = np . This yields
Ã
me kT
1−X
= np
X
2πh̄2
!−3/2
Q
exp
kT
µ
¶
.
(9.25)
To eliminate np from the above equation, we recall that η ≡ nbary /nγ . In
a universe where hydrogen is the only element, and a fraction X of the
hydrogen is in the form of naked protons, we may write
η=
9
np
.
Xnγ
(9.26)
If you are a true thermodynamic maven, you will have noted that equation (9.21) omits
the chemical potential term, µ, which appears in the most general form of the MaxwellBoltzmann equation. In most cosmological contexts, as it turns out, the chemical potential
is small enough to be safely neglected.
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Since the photons have a blackbody spectrum, for which
2.404
nγ =
π2
Ã
kT
h̄c
!3
Ã
kT
= 0.243
h̄c
!3
,
(9.27)
we can combine equations (9.26) and (9.27) to find
Ã
kT
np = 0.243Xη
h̄c
!3
.
(9.28)
Substituting equation (9.28) back into equation (9.25), we finally find the
desired equation for X in terms of T and η:
Ã
1−X
kT
= 3.84η
2
X
me c2
!3/2
Q
exp
kT
µ
¶
.
This is a quadratic equation in X, whose positive root is
√
−1 + 1 + 4S
,
X=
2S
where
Ã
kT
S(T, η) = 3.84η
me c2
!3/2
exp
µ
Q
kT
¶
(9.29)
(9.30)
.
(9.31)
If we define the moment of recombination as the exact instant when X = 1/2,
then (assuming η = 5.5 × 10−10 ) the recombination temperature is
kTrec = 0.323 eV =
Q
.
42
(9.32)
Because of the exponential dependence of S upon the temperature, the exact
value of η doesn’t strongly affect the value of Trec . In degrees Kelvin, kTrec =
0.323 eV corresponds to a temperature Trec = 3740 K, slightly higher than
the melting point of tungsten.10 The temperature of the universe had a value
T = Trec = 3740 K at a redshift zrec = 1370, when the age of the universe,
in the Benchmark Model, was trec = 240,000 yr. Recombination was not an
instantaneous process; however, as shown in Figure 9.4, it proceeded fairly
rapidly. The fractional ionization goes from X = 0.9 at a redshift z = 1475
10
Not that there was any tungsten around back then to be melted.
9.3. THE PHYSICS OF RECOMBINATION
193
1
X
.8
.6
.4
.2
0
1200 1300 1400 1500 1600
z
Figure 9.4: The fractional ionization X as a function of redshift during the
epoch of recombination. A baryon-to-photon ratio of η = 5.5 × 10−10 is
assumed.
to X = 0.1 at a redshift z = 1255. In the Benchmark Model, the time that
elapses from X = 0.9 to X = 0.1 is ∆t ≈ 70,000 yr.
Since the number density of free electrons drops rapidly during the epoch
of recombination, the time of photon decoupling comes soon after the time of
recombination. The rate of photon scattering, when the hydrogen is partially
ionized, is
Γ(z) = ne (z)σe c = X(z)(1 + z)3 nbary,0 σe c .
(9.33)
Using Ωbary,0 = 0.04, the numerical value of the scattering rate is
Γ(z) = 4.4 × 10−21 s−1 X(z)(1 + z)3 .
(9.34)
While recombination is taking place, the universe is matter-dominated, so
the Hubble parameter is given by the relation
Ωm,0
H2
= 3 = Ωm,0 (1 + z)3 .
2
H0
a
(9.35)
Using Ωm,0 = 0.3, the numerical value of the Hubble parameter during the
epoch of recombination is
H(z) = 1.24 × 10−18 s−1 (1 + z)3/2 .
(9.36)
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CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
The redshift of photon decoupling is found by setting Γ = H, or (combining
equations 9.34 and 9.36),
43.0
.
X(zrec )2/3
1 + zrec =
(9.37)
Using the value of X(z) given by the Saha equation (shown in Figure 9.4),
the redshift of photon decoupling is found to be zdec = 1130. In truth, the
exact redshift of photon decoupling is somewhat smaller than this value. The
Saha equation assumes that the reaction H + γ *
) p + e− is in equilibrium.
However, when Γ starts to drop below H, the photoionization reaction is
no longer in equilibrium. As a consequence, at redshifts smaller than ∼
1200, the fractional ionization X is larger than would be predicted by the
Saha equation, and the decoupling of photons is therefore delayed. Without
going into the details of the non-equilibrium physics, let’s content ourselves
by saying, in round numbers, zdec ≈ 1100, corresponding to a temperature
Tdec ≈ 3000 K, when the age of the universe was tdec ≈ 350,000 yr in the
Benchmark Model.
When we examine the CMB with our microwave antennas, the photons
we collect have been traveling straight toward us since the last time they
scattered from a free electron. During a brief time interval t → t + dt, the
probability that a photon undergoes a scattering is dP = Γ(t)dt, where Γ(t)
is the scattering rate at time t. Thus, if we detect a CMB photon at time t 0 ,
the expected number of scatterings it has undergone since an earlier time t
is
Z t0
τ (t) =
Γ(t)dt .
(9.38)
t
The dimensionless number τ is the optical depth. The time t for which τ = 1
is the time of last scattering, and represents the time which has elapsed since
a typical CMB photon last scattered from a free electron. If we change the
variable of integration in equation (9.38) from t to a, we find that
τ (a) =
Z
1
a
Γ(a)
da Z 1 Γ(a) da
=
,
ȧ
a H(a) a
(9.39)
using the fact that H = ȧ/a. Alternatively, we can find the optical depth as
a function of redshift by making the substitution 1 + z = 1/a:
τ (z) =
Z
z
0
Z z
Γ(z) dz
= 0.0035
X(z)(1 + z)1/2 dz .
H(z) 1 + z
0
(9.40)
9.3. THE PHYSICS OF RECOMBINATION
Table 9.1: Events in the early universe
event
redshift temperature (K) time
radiation-matter equality
3570
9730
recombination
1370
3740
photon decoupling
1100
3000
last scattering
1100
3000
195
(megayears)
0.047
0.24
0.35
0.35
Here, I have made use of equations (9.34) and (9.36).11 As it turns out, the
last scattering of a typical CMB photon occurs after the photoionization reaction H + γ *
) p + e− falls out of equilibrium, so the Saha equation doesn’t
strictly apply. To sufficient accuracy for our purposes, we can state that the
redshift of last scattering was comparable to the redshift of photon decoupling: zls ≈ zdec ≈ 1100. Not all the CMB photons underwent their last
scattering simultaneously; the universe doesn’t choreograph its microphysics
that well. If you scoop up two photons from the CMB, one may have undergone its last scattering at z = 1200, while the other may have scattered more
recently, at z = 1000. Thus, the “last scattering surface” is really more of a
“last scattering layer”; just as we can see a little way into a fog bank here on
Earth, we can see a little way into the “electron fog” which hides the early
universe from our direct view.
The relevant times of various events around the time of recombination
are shown in Table 9.1. For purposes of comparison, the table also contains
the time of radiation-matter equality, emphasizing the fact that recombination, photon decoupling, and last scattering took place when the universe
was matter-dominated. Note that all these times are approximate, and are
dependent on the cosmological model you choose. (I have chosen the Benchmark Model in calculating these numbers.) When we look at the CMB, we
are getting an intriguing glimpse of the universe as it was when it was only
1/40,000 of its present age.
The epoch of photon decoupling marked an important change in the state
of the universe. Before photon decoupling, there existed a single photonbaryon fluid, consisting of photons, electrons, and protons coupled together.
Since the photons traveled about at the speed of light, kicking the electrons
11
By the time the universe becomes Λ dominated, the free electron density has fallen to
negligibly small levels, so using the Hubble parameter for a matter-dominated universe is
a justifiable approximation in computing τ .
196
CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
before them as they went, they tended to smooth out any density fluctuations in the photon-baryon fluid smaller than the horizon. After photon
decoupling, however, the photon-baryon fluid became a pair of gases, one of
photons and the other of neutral hydrogen. Although the two gases coexisted
spatially, they were no longer coupled together. Thus, instead of being kicked
to and fro by the photons, the hydrogen gas was free to collapse under its
own self-gravity (and the added gravitational attraction of the dark matter).
Thus, when we look at the Cosmic Microwave Background, we are looking
backward in time to an important epoch in the history of the universe – the
epoch when the baryons, free from the harassment of photons, were free to
collapse gravitationally, and thus form galaxies, stars, planets, cosmologists,
and the other dense knots of baryonic matter which make the universe so
rich and strange today.
9.4
Temperature fluctuations
The dipole distortion of the Cosmic Microwave Background, shown in the top
panel of Figure 9.2, results from the fact that the universe is not perfectly
homogeneous today (at z = 0). Because we are gravitationally accelerated
towards the nearest large lumps of matter, we see a Doppler shift in the
radiation of the CMB. The distortions on a smaller angular scale, shown in
the bottom panel of Figure 9.2, tell us that the universe was not perfectly
homogeneous at the time of last scattering (at z ≈ 1100). The angular size of
the temperature fluctuations reflects in part the physical size of the density
and velocity fluctuations at z ≈ 1100. The COBE DMR experiment had limited angular resolution, and was only able to detect temperature fluctuations
larger than δθ ≈ 7◦ . More recent experiments have provided higher angular
resolution. For instance, MAXIMA (a balloon-borne experiment), DASI (an
experiment located at the South Pole), and BOOMERANG (a balloon-borne
experiment launched from Antarctica), all have provided maps of δT /T down
to scales of δθ ∼ 10 arcminutes.
The angular size δθ of a temperature fluctuation in the CMB is related
to a physical size ` on the last scattering surface by the relation
`
,
(9.41)
δθ
where dA is the angular-diameter distance to the last scattering surface.
Since the last scattering surface is at a redshift zls = 1100 À 1, a good
dA =
197
9.4. TEMPERATURE FLUCTUATIONS
approximation to dA is given by equation (7.41):
dA ≈
dhor (t0 )
.
zls
(9.42)
In the Benchmark Model, the current horizon distance is dhor (t0 ) ≈ 14,000 Mpc,
so the angular-diameter distance to the surface of last scattering is
dA ≈
14,000 Mpc
≈ 13 Mpc .
1100
(9.43)
Thus, fluctuations on the last scattering surface with an observed angular
size δθ had a proper size
Ã
δθ
` = dA (δθ) = 13 Mpc
1 rad
!
Ã
δθ
= 0.22 Mpc ◦
1
!
(9.44)
at the time of last scattering. Thus, the fluctuations that gave rise to the fluctuations seen by COBE (with δθ > 7◦ ) had a proper size ` > 1.6 Mpc. However, the fluctuations at the time of last scattering were not gravitationally
bound objects; they were expanding along with the universal Hubble expansion. Thus, the fluctuations seen by COBE correspond to physical scales of
`(1 + zls ) > 1700 Mpc today, much larger than the biggest superclusters. The
higher-resolution experiments such as MAXIMA, DASI, and BOOMERANG
see fluctuations corresponding to scales as small as ` ≈ 0.04 Mpc at the time
of last scattering, or `(1 + zls ) ≈ 40 Mpc today, about the size of today’s
superclusters.
Consider the density fluctuations δT /T observed by a particular experiment. Figure 9.5, for instance, shows δT /T as measured by COBE at low
resolution over the entire sky, and as measured by BOOMERANG at higher
resolution over part of the sky. Since δT /T is defined on the surface of a
sphere – the celestial sphere, in this case – it is useful to expand it in spherical harmonics:
∞ X
l
X
δT
(θ, φ) =
alm Ylm (θ, φ) ,
(9.45)
T
l=0 m=−l
where Ylm (θ, φ) are the usual spherical harmonic functions. What concerns
cosmologists is not the exact pattern of hot spots and cold spots on the
sky, but their statistical properties. The most important statistical property
of δT /T is the correlation function C(θ). Consider two points on the last
198
CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Figure 9.5: Upper left: the temperature fluctuations measured by the COBE
DMR instrument, after the dipole and galaxy foreground are subtracted.
Lower right: the temperature fluctuations measured by the BOOMERANG
experiment over a region ∼ 20◦ across. The white spots are regions 300 microKelvin warmer than average; the black spots are regions 300 microKelvin
cooler than average. (Courtesy of the BOOMERANG collaboration.)
9.4. TEMPERATURE FLUCTUATIONS
199
scattering surface. Relative to an observer, they are in the directions n̂ and
n̂0 , and are separated by an angle θ given by the relation cos θ = n̂ · n̂0 . To
find the correlation function C(θ), multiply together the values of δT /T at
the two points, then average the product over all points separated by the
angle θ:
+
*
δT 0
δT
(n̂) (n̂ )
.
(9.46)
C(θ) =
T
T
n̂·n̂0 =cos θ
If cosmologists knew the precise value of C(θ) for all angles from θ = 0 to θ =
180◦ , they would have a complete statistical description of the temperature
fluctuations over all angular scales. Unfortunately, the CMB measurements
which tell us about C(θ) contain information over only a limited range of
angular scales.
The limited angular resolution of available observations is what makes the
spherical harmonic expansion of δT /T , shown in equation (9.45), so useful.
Using the expansion of δT /T in spherical harmonics, the correlation function
can be written in the form
∞
1 X
(2l + 1)Cl Pl (cos θ) ,
C(θ) =
4π l=0
(9.47)
where Pl are the usual Legendre polynomials:
P0 (x) = 1
P1 (x) = x
1
P2 (x) =
(3x2 − 1)
2
(9.48)
and so forth. In this way, a measured correlation function C(θ) can be broken
down into its multipole moments Cl . For a given experiment, the value of Cl
will be nonzero for angular scales larger than the resolution of the experiment
and smaller than the patch of sky examined. Generally speaking, a term Cl is
a measure of temperature fluctuations on the angular scale θ ∼ 180◦ /l. Thus,
the multipole l is interchangeable, for all practical purposes, with the angular
scale θ. The l = 0 (monopole) term of the correlation function vanishes if
you’ve defined the mean temperature correctly. The l = 1 (dipole) term
results primarily from the Doppler shift due to our motion through space. It
is the moments with l ≥ 2 which are of the most interest to cosmologists,
since they tell us about the fluctuations present at the time of last scattering.
200
CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Figure 9.6: The anisotropy of the CMB temperature, ∆T , expressed as a
function of the multipole l (from Wang, Tegmark, & Zaldarriaga, 2002, Phys.
Rev. D).
In presenting the results of CMB observations, it is customary to plot the
function
Ã
!1/2
l(l + 1)
Cl
∆T ≡
hT i
(9.49)
2π
since this function tells us the contribution per logarithmic interval in l to
the total temperature fluctuation δT of the Cosmic Microwave Background.
Figure 9.6, which combines data from a large number of CMB experiments,
is a plot of ∆T as a function of the logarithm of l. Note that the temperature
fluctuation has a peak at l ∼ 200, corresponding to an angular size of ∼ 1◦ .
The detailed shape of the ∆T versus l curve, as shown in Figure 9.6, contains
a wealth of information about the universe at the time of photon decoupling.
In the next section, we will examine, very briefly, the physics behind the
temperature fluctuations, and how we can extract cosmological information
from the temperature anisotropy of the Cosmic Microwave Background.
9.5. WHAT CAUSES THE FLUCTUATIONS?
9.5
201
What causes the fluctuations?
At the time of last scattering, a particularly interesting length scale, cosmologically speaking, is the Hubble distance,
c/H(zls ) ≈
3.0 × 108 m s−1
≈ 6.6 × 1021 m ≈ 0.2 Mpc ,
1.24 × 10−18 s−1 (1101)3/2
(9.50)
where I have used equation (9.36) to compute the Hubble parameter at the
redshift of last scattering, zls ≈ 1100. A patch of the last scattering surface
with this physical size will have an angular size, as seen from Earth, of
θH =
c/H(zls )
0.2 Mpc
≈ 0.015 rad ≈ 1◦ .
≈
dA
13 Mpc
(9.51)
It is no coincidence that the peak in the ∆T versus l curve (Figure 9.6)
occurs at an angular scale θ ∼ θH . The origin of temperature fluctuations
with θ > θH (l < 180) is different from those with θ < θH (l > 180).
Consider first the large-scale fluctuations – those with angular size θ > θH .
These temperature fluctuations arise from the gravitational effect of primordial density fluctuations in the distribution of nonbaryonic dark matter. The
density of nonbaryonic dark matter at the time of last scattering was
εdm (zls ) = Ωdm,0 εc,0 (1 + zls )3
(9.52)
since the energy density of matter is ∝ a−3 ∝ (1 + z)3 . Plugging in the
appropriate numbers, we find that
εdm (zls ) ≈ (0.26)(5200 MeV m−3 )(1101)3 ≈ 1.8 × 1012 MeV m−3 ,
(9.53)
equivalent to a mass density of ∼ 3 × 10−18 kg m−3 . The density of baryonic
matter at the time of last scattering was
εbary (zls ) = Ωbary,0 ²c,0 (1 + zls )3 ≈ 2.8 × 1011 MeV m−3 .
(9.54)
The density of photons at the time of last scattering, since εγ ∝ a−4 ∝
(1 + z)4 , was
εγ (zls ) = Ωγ,0 ²c,0 (1 + zls )4 ≈ 3.8 × 1011 MeV m−3 .
(9.55)
Thus, at the time of last scattering, εdm > εγ > εbary , with dark matter,
photons, and baryons having energy densities in roughly the ratio 6.4 : 1.4 : 1.
202
CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
The nonbaryonic dark matter dominated the energy density ², and hence the
gravitational potential, of the universe at the time of last scattering.
Suppose that the density of the nonbaryonic dark matter at the time of
last scattering was not perfectly homogeneous, but varied as a function of
position. Then we could write the energy density of the dark matter as
ε(~r) = ε̄ + δε(~r) ,
(9.56)
where ε̄ is the spatially averaged energy density of the nonbaryonic dark
matter, and δε is the local deviation from the mean. In the Newtonian
approximation, the spatially varying component of the energy density, δε,
gives rise to a spatially varying gravitational potential δΦ. The link between
δε and δΦ is Poisson’s equation:
∇2 (δΦ) =
4πG
δε .
c2
(9.57)
Unless the distribution of dark matter were perfectly smooth at the time of
last scattering, the fluctuations in its density would necessarily have given
rise to fluctuations in the gravitational potential.
Consider the fate of a CMB photon which happens to be at a local minimum of the potential at the time of last scattering. (Minima in the gravitational potential are known colloquially as “potential wells”.) In climbing
out of the potential well, it loses energy, and consequently is redshifted.
Conversely, a photon which happens to be at a potential maximum when
the universe became transparent gains energy as it falls down the “potential hill”, and thus is blueshifted. The cool (redshifted) spots on the COBE
temperature map correspond to minima in δΦ at the time of last scattering;
the hot (blueshifted) spots correspond to maxima in δΦ. A detailed general
relativistic calculation, first performed by Sachs and Wolfe in 1967, tells us
that
δT
1 δΦ
=
.
(9.58)
T
3 c2
Thus, the temperature fluctuations on large angular scales (θ > θH ≈ 1◦ )
give us a map of the potential fluctuations δΦ present at the time of last
scattering. The creation of temperature fluctuations by variations in the
gravitational potential is known as the Sachs-Wolfe effect, in tribute to the
work of Sachs and Wolfe.
On smaller scales (θ < θH ), the origin of the temperature fluctuations
in the CMB is complicated by the behavior of the photons and baryons.
9.5. WHAT CAUSES THE FLUCTUATIONS?
203
Consider the situation immediately prior to photon decoupling. The photons,
electrons, and protons together make a single photon-baryon fluid, whose
energy density is only about a third that of the dark matter. Thus, the
photon-baryon fluid moves primarily under the gravitational influence of the
dark matter, rather than under its own self-gravity. The equation-of-state
parameter wpb of the photon-baryon fluid is intermediate between the value
w = 1/3 expected for a gas containing only photons and the value w = 0
expected for a gas containing only cold baryons and electrons. If the photonbaryon fluid finds itself in a potential well of the dark matter, it will fall to
the center of the well.12 As the photon-baryon fluid is compressed by gravity,
however, its pressure starts to rise. Eventually, the pressure is sufficient to
cause the fluid to expand outward. As the expansion continues, the pressure
drops until gravity causes the photon-baryon fluid to fall inward again. The
cycle of compression and expansion which is set up continues until the time
of photon decoupling. The inward and outward oscillations of the photonbaryon fluid are called acoustic oscillations, since they represent a type of
standing sound wave in the photon-baryon fluid.
If the photon-baryon fluid within a potential well is at maximum compression at the time of photon decoupling, its density will be higher than
average, and the liberated photons, since T ∝ ε1/4 , will be hotter than average. Conversely, if the photon-baryon fluid within a potential well is at
maximum expansion at the time of decoupling, the liberated photons will be
slightly cooler than average. If the photon-baryon fluid is in the process of
expanding or contracting at the time of decoupling, the Doppler effect will
cause the liberated photons to be cooler or hotter than average, depending
on whether the photon-baryon fluid was moving away from our location or
toward it at the time of photon decoupling. Computing the exact shape of
the ∆T versus l curve expected in a particular model universe is a rather
complicated chore. Generally speaking, however, the highest peak in the ∆T
curve (at l ∼ 200 or θ ∼ 1◦ in Figure 9.6) represents the potential wells
within which the photon-baryon fluid had just reached maximum compression at the time of last scattering. These potential wells had proper sizes
∼ c/H(zls ) at the time of last scattering, and hence have angular sizes of
12
Note that if the size of the well is larger than c/H(zls ), the photon-baryon fluid, which
travels at a speed < c, will not have time to fall to the center by the time of last scattering
tls ∼ 1/H(zls ). This is why the motions of the photons and baryons are irrelevant on
scales θ > θH , and why the temperature fluctuations on these large scales are dictated
purely by the distribution of dark matter.
204
CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
∼ θH as seen by us at the present day.
The location and amplitude of the highest peak in Figure 9.6 is a very
useful cosmological diagnostic. As it turns out, the angle θ ≈ 1◦ at which it’s
located is dependent on the spatial curvature of the universe. In a negatively
curved universe (κ = −1), the angular size θ of an object of known proper
size at a known redshift is smaller than it is in a positively curved universe
(κ = +1). If the universe were negatively curved, the peak in ∆T would be
seen at an angle θ < 1◦ , or l > 180; if the universe were positively curved, the
peak would be seen at an angle θ > 1◦ , or l < 180. The observed position of
the peak is consistent with κ = 0, or Ω0 = 1. Figure 9.7 shows the values of
Ωm,0 and ΩΛ,0 which are permitted by the present CMB data. Note that the
shaded areas which show the best fit to the CMB data are roughly parallel to
the Ω = 1 line (representing κ = 0 universes) and roughly perpendicular to
the region permitted by the type Ia supernova results. The oval curve marked
“68%” in Figure 9.7 represents the region consistent with both the CMB
results and the supernova results. A spatially flat universe, with Ωm,0 ≈ 0.3
and ΩΛ,0 ≈ 0.7, agrees with the CMB results, the supernova results, and
the computed density of matter in clusters (as discussed in Chapter 8). This
happy concurrence is the basis for the “Benchmark Model” which I have been
using in this book.
The angular size corresponding to the highest peak in the ∆T versus l
curve gives useful information about the density parameter Ω and the curvature κ. The amplitude of the peak is dependent on the sound speed of the
photon-baryon fluid prior to photon decoupling. Since the sound speed of
the photon-baryon fluid is
√
cs = wpb c ,
(9.59)
and the equation-of-state parameter wpb is in turn dependent on the baryonto-photon ratio, the amplitude of the peak is a useful diagnostic of the baryon
density of the universe. Detailed analysis of the ∆T curve, with the currently
available data, yields
Ωbary,0 = 0.04 ± 0.02 ,
(9.60)
assuming a Hubble constant of H0 = 70 km s−1 Mpc−1 . As we’ll see in the
next chapter, this baryon density is consistent with that found from the
entirely different arguments of primordial nucleosynthesis.
9.5. WHAT CAUSES THE FLUCTUATIONS?
205
Figure 9.7: The values of Ωm,0 and ΩΛ,0 permitted by the MAXIMA and
COBE DMR data. The light gray area near the Ω = 1 line shows the bestfitting values of Ωm,0 and ΩΛ,0 . The SN Ia data (as seen earlier in Figure 7.6)
are also shown. The curves marked 68%, 95%, and 98% give the best fits,
at the designated confidence levels, for the combined CMB and SN Ia data
(from Stompor et al. 2001, ApJ, 561, L7).
206
CHAPTER 9. THE COSMIC MICROWAVE BACKGROUND
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Coles (1999), Part I, ch. 5: An insightful overview of the origin of the
temperature anisotropies in the CMB.
Liddle (1999), ch. 9: A simplified overview of photon decoupling and the
origin of the CMB.
Rich (2001), ch. 7.9: A discussion of the anisotropies in the CMB, and
their use as diagnostics of cosmological parameters.
Problems
(9.1) The purpose of this problem is to determine how the uncertainty in
the value of the baryon-to-photon ratio, η, affects the recombination
temperature in the early universe. Plot the fractional ionization X
as a function of temperature, in the range 3000 K < T < 4500 K;
first make the plot assuming η = 4 × 10−10 , then assuming η = 8 ×
10−10 . How much does this change in η affect the computed value of the
recombination temperature Trec , if we define Trec as the temperature at
which X = 1/2?
(9.2) Suppose the temperature T of a blackbody distribution is such that
kT ¿ Q, where Q = 13.6 eV is the ionization energy of hydrogen.
What fraction f of the blackbody photons are energetic enough to
ionize hydrogen? If T = Trec = 3740 K, what is the numerical value of
f?
(9.3) Imagine that at the time of recombination, the baryonic portion of the
universe consisted entirely of 4 He (that is, helium with two protons and
two neutrons in its nucleus). The ionization energy of helium (that is,
the energy required to convert neutral He to He+ ) is QHe = 24.6 eV.
At what temperature would the fractional ionization of the helium be
X = 1/2? Assume that η = 5.5 × 10−10 and that the number density
of He++ is negligibly small. (The relevant statistical weight factor for
the ionization of helium is gHe /(ge gHe+ ) = 1/4.)
9.5. WHAT CAUSES THE FLUCTUATIONS?
207
(9.4) What is the proper distance dp to the surface of last scattering? What
is the luminosity distance dL to the surface of last scattering? Assume
that the Benchmark Model is correct, and that the redshift of the last
scattering surface is zls = 1100.
(9.5) We know from observations that the intergalactic medium is currently
ionized. Thus, at some time between trec and t0 , the intergalactic
medium must have been reionized. The fact that we can see small
fluctuations in the CMB places limits on how early the reionization
took place. Assume that the baryonic component of the universe instantaneously became completely reionized at some time t∗ . For what
value of t∗ does the optical depth of reionized material,
τ=
Z
t0
t∗
Γ(t)dt =
Z
t0
t∗
ne (t)σe cdt ,
(9.61)
equal one? For simplicity, assume that the universe is spatially flat and
matter-dominated, and that the baryonic component of the universe is
pure hydrogen. To what redshift z∗ does this value of t∗ correspond?
Chapter 10
Nucleosynthesis & the Early
Universe
The Cosmic Microwave Background tells us a great deal about the state
of the universe at the time of last scattering (tls ≈ 0.35 Myr). However,
the opacity of the early universe prevents us from directly seeing what the
universe was like at t < tls . Photons are the “messenger boys” of astronomy;
although some information is carried by cosmic rays and by neutrinos, most
of what we know about the universe beyond our Solar System has come in
the form of photons. Looking at the last scattering surface is like looking
at the surface of a cloud, or the surface of the Sun; our curiosity is piqued,
and we wish to find out what conditions are like in the opaque regions so
tantalizingly hidden from our direct view.
From a theoretical viewpoint, many properties of the early universe should
be quite simple. For instance, when radiation is strongly dominant over
matter, at scale factors a ¿ arm ≈ 2.8 × 10−4 , or times t ¿ trm ≈ 47,000 yr,
the expansion of the universe has the simple power-law form a(t) ∝ t1/2 . The
temperature of the blackbody photons in the early universe, which decreases
as T ∝ a−1 as the universe expands, is given by the convenient relation
t
T (t) ≈ 10 K
1s
10
µ
¶−1/2
,
(10.1)
or equivalently
kT (t) ≈ 1 MeV
208
µ
t
1s
¶−1/2
.
(10.2)
209
10.1. NUCLEAR PHYSICS AND COSMOLOGY
Thus the mean energy per photon was
t
Emean (t) ≈ 2.7kT (t) ≈ 3 MeV
1s
µ
¶−1/2
.
(10.3)
The Fermi National Accelerator Laboratory, in Batavia, Illinois, is justly
proud of its “Tevatron”, designed to accelerate protons to an energy of 1 TeV;
that’s 1012 eV, or a thousand times the rest energy of a proton. Well, when
the universe was one picosecond old (t = 10−12 s), the entire universe was a
Tevatron. Thus, the early universe is referred to as “the poor man’s particle
accelerator”, since it provided particles of very high energy without running
up an enormous electricity bill or having Congress threaten to cut off its
funding.
10.1
Nuclear physics and cosmology
As the universe has expanded and cooled, the mean energy per photon has
dropped from Emean (tP ) ∼ EP ∼ 1028 eV at the Planck time to Emean (t0 ) ≈
6 × 10−4 eV at the present day. Thus, by studying the universe as it expands,
we sample over 31 orders of magnitude in particle energy. Within this wide
energy range, some energies are of more interest than others to physicists. For
instance, to physicists studying recombination and photoionization, the most
interesting energy scale is the ionization energy of an atom. The ionization
energy of hydrogen is Q = 13.6 eV, as we have already noted. The ionization
energies of other elements (that is, the energy required to remove the most
loosely bound electron in the neutral atom) are roughly comparable. Thus,
atomic physicists, when considering the ionization of atoms, typically deal
with energies of ∼ 10 eV, in round numbers.
Nuclear physicists are concerned not with ionization and recombination
(removing or adding electrons to an atom), but with the much higher energy
processes of fission and fusion (splitting or merging atomic nuclei). An atomic
nucleus contains Z protons and N neutrons, where Z ≥ 1 and N ≥ 0.
Protons and neutrons are collectively called nucleons. The total number of
nucleons within an atomic nucleus is called the mass number, and is given by
the formula A = Z + N . The proton number Z of a nucleus determines the
atomic element to which that nucleus belongs. For instance, hydrogen (H)
nuclei all have Z = 1, helium (He) nuclei have Z = 2, lithium (Li) nuclei have
Z = 3, beryllium (Be) nuclei have Z = 4, and so on, through the complete
210
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
periodic table. Although all atoms of a given element have the same number
of protons in their nuclei, different isotopes of an element can have different
numbers of neutrons. A particular isotope of an element is designated by
prefixing the mass number A to the symbol for that element. For instance, a
standard hydrogen nucleus, with one proton and no neutrons, is symbolized
as 1 H. (Since an ordinary hydrogen nucleus is nothing more than a proton,
we may also write p in place of 1 H when considering nuclear reactions.)
Heavy hydrogen, or deuterium, contains one proton and one neutron, and
is symbolized as 2 H. (Since deuterium is very frequently mentioned in the
context of nuclear fusion, it has its own special symbol, D.) Ordinary helium
contains two protons and two neutrons, and is symbolized as 4 He.
The binding energy B of a nucleus is the energy required to pull it apart
into its component protons and neutrons. Equivalently, it is the energy released when a nucleus is fused together from individual protons and neutrons.
For instance, when a neutron and a proton are bound together to form a deuterium nucleus, an energy of BD = 2.22 MeV is released:
p+n*
) D + 2.22 MeV .
(10.4)
The deuterium nucleus is not very tightly bound, compared to other atomic
nuclei. Figure 10.1 plots the binding energy per nucleon (B/A) for atomic
nuclei with different mass numbers. Note that 4 He, with a total binding
energy of B = 28.30 MeV, and a binding energy per nucleon of B/A =
7.07 MeV, is relatively tightly bound, compared to other light nuclei (that
is, nuclei with A ≤ 10). The most tightly bound nuclei are those of 56 Fe and
62
Ni, which both have B/A ≈ 8.8 MeV. Thus, nuclei more massive than iron
or nickel can release energy by fission – splitting into lighter nuclei. Nuclei
less massive than iron or nickel can release energy by fusion – merging into
heavier nuclei.
Thus, just as studies of ionization and recombination deal with an energy
scale of ∼ 10 eV (a typical ionization energy), so studies of nuclear fusion and
fission deal with an energy scale of ∼ 8 MeV (a typical binding energy per
nucleon). Moreover, just as electrons and protons combined to form neutral
hydrogen atoms when the temperature dropped sufficiently far below the ionization energy of hydrogen (Q = 13.6 eV), so protons and neutrons must have
fused to form deuterium when the temperature dropped sufficiently far below
the binding energy of deuterium (BD = 2.22 MeV). The epoch of recombination must have been preceded by an epoch of nuclear fusion, commonly called
211
10.1. NUCLEAR PHYSICS AND COSMOLOGY
B/A (MeV)
8
4
He
6
4
3
H
3
He
2
D
0
1
10
A
100
Figure 10.1: The binding energy per nucleon (B/A) as a function of the
number of nucleons (protons and neutrons) in an atomic nucleus. Note the
absence of nuclei at A = 5 and A = 8.
212
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
the epoch of Big Bang Nucleosynthesis (BBN). Nucleosynthesis in the early
universe starts by the fusion of neutrons and protons to form deuterium, then
proceeds to form heavier nuclei by successive acts of fusion. Since the binding
energy of deuterium is larger than the ionization energy of hydrogen by a factor BD /Q = 1.6 × 105 , we would expect, as a rough estimate, the synthesis of
deuterium to occur at a temperature 1.6×105 times higher than the recombination temperature Trec = 3740 K. That is, deuterium synthesis occurred at
a temperature Tnuc ≈ 1.6 × 105 (3740 K) ≈ 6 × 108 K, corresponding to a time
tnuc ≈ 300 s. This estimate, as we’ll see when we do the detailed calculations,
gives a temperature slightly too low, but it certainly gives the right order of
magnitude. As indicated in the title of Steven Weinberg’s classic book, The
First Three Minutes, the entire saga of Big Bang Nucleosynthesis takes place
when the universe is only a few minutes old.
One thing we can say about Big Bang Nucleosynthesis, after taking a look
at the present-day universe, is that it was shockingly inefficient. From an
energy viewpoint, the preferred universe would be one in which the baryonic
matter consisted of an iron-nickel alloy. Obviously, we do not live in such a
universe. Currently, three-fourths of the baryonic component (by mass) is
still in the form of unbound protons, or 1 H. Moreover, when we look for nuclei
heavier than 1 H, we find that they are primarily 4 He, a relatively lightweight
nucleus compared to 56 Fe and 62 Ni. The primordial helium fraction of the
universe (that is, the helium fraction before nucleosynthesis begins in stars)
is usually expressed as the dimensionless number
Yp ≡
ρ(4 He)
.
ρbary
(10.5)
That is, Yp is the mass density of 4 He divided by the mass density of all
the baryonic matter. The Sun’s atmosphere has a helium fraction (by mass)
of Y = 0.28. However, the Sun is made of recycled interstellar gas, which
was contaminated by helium formed in earlier generations of stars. When we
look at astronomical objects of different sorts, we find a minimum value of
Y = 0.24. That is, baryonic objects such as stars and gas clouds are all at
least 24% helium.1
1
Condensed objects which have undergone chemical or physical fractionation can be
much lower in helium than this value. For instance, your helium fraction is ¿ 24%.
10.2. NEUTRONS AND PROTONS
10.2
213
Neutrons and protons
The basic building blocks for nucleosynthesis are neutrons and protons. The
rest energy of a neutron is greater than that of a proton by a factor
Qn = mn c2 − mp c2 = 1.29 MeV .
(10.6)
A free neutron is unstable, decaying via the reaction
n → p + e− + ν̄e .
(10.7)
The decay time for a free neutron is τn = 890 s. That is, if you start out
with a population of free neutrons, after a time t, a fraction f = exp(−t/τ n )
will remain.2 Since the energy Qn released by the decay of a neutron into a
proton is greater than the rest energy of an electron (me c2 = 0.51 MeV), the
remainder of the energy is carried away by the kinetic energy of the electron
and the energy of the electron anti-neutrino. With a decay time of only
fifteen minutes, the existence of a free neutron is as fleeting as fame; once
the universe was several hours old, it contained essentially no free neutrons.
However, a neutron which is bound into a stable atomic nucleus is preserved
against decay. There are still neutrons around today, because they’ve been
tied up in deuterium, helium, and other atoms.
Let’s consider the state of the universe when its age is t = 0.1 s. At that
time, the temperature was T ≈ 3 × 1010 K, and the mean energy per photon
was Emean ≈ 10 MeV. This energy is much greater than the rest energy of a
electron or positron, so there were positrons as well as electrons present at
t = 0.1 s, created by pair production:
γ+γ *
) e− + e + .
(10.8)
At t = 0.1 s, neutrons and protons were in equilibrium with each other, via
the interactions
n + νe *
(10.9)
) p + e−
and
n + e+ *
) p + ν̄e .
(10.10)
As long as neutrons and protons are kept in equilibrium by the reactions
shown in equations (10.9) and (10.10), their number density is given by
2
The ‘half-life’, the time it takes for half the neutrons to decay, is related to the decay
time by the relation t1/2 = τn ln 2 = 617 s.
214
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
the Maxwell-Boltzmann equation, as discussed in section 9.3. The number
density of neutrons is then
nn = g n
Ã
mn kT
2πh̄2
!3/2
mn c2
exp −
kT
Ã
!
(10.11)
and the number density of protons is
np = g p
Ã
mp kT
2πh̄2
!3/2
mp c2
exp −
kT
Ã
!
.
(10.12)
Since the statistical weights of protons and neutrons are equal, with gp =
gn = 2, the neutron-to-proton ratio, from equations (10.11) and (10.12), is
nn
=
np
Ã
mn
mp
!3/2
(mn − mp )c2
exp −
kT
Ã
!
.
(10.13)
The above equation can be simplified. First, (mn /mp )3/2 = 1.002; there will
be no great loss in accuracy if we set this factor equal to one. Second, the
difference in rest energy of the neutron and proton is (mn − mp )c2 = Qn =
1.29 MeV. Thus, the equilibrium neutron-to-proton ratio has the particularly
simple form
¶
µ
nn
Qn
,
(10.14)
= exp −
np
kT
illustrated as the solid line in Figure 10.2. At temperatures kT À Qn =
1.29 MeV, corresponding to T À 1.5 × 1010 K and t ¿ 1 s, the number of
neutrons is nearly equal to the number of protons. However, as the temperature starts to drop below 1.5 × 1010 K, protons begin to be strongly favored,
and the neutron-to-proton ratio plummets exponentially.
If the neutrons and protons remained in equilibrium, then by the time
the universe was six minutes old, there would be only one neutron for every
million protons. However, neutrons and protons do not remain in equilibrium
for nearly that long. The interactions which mediate between neutrons and
protons in the early universe, shown in equations (10.9) and (10.10), involve
the interaction of a baryon with a neutrino (or anti-neutrino). Neutrinos
interact with baryons via the weak nuclear force. The cross-sections for weak
interactions have the temperature dependence σw ∝ T 2 ; at the temperatures
we are considering, the cross-sections are small. A typical cross-section for
215
10.2. NEUTRONS AND PROTONS
1
freezeout
.1
nn/np
.01
.001
.0001
10
−5
10
−6
100
10
9
T (10 K)
1
Figure 10.2: Neutron-to-proton ratio in the early universe. The solid line
assumes equilibrium; the dotted line gives the value after freezeout.
the interaction of a neutrino with any other particle via the weak nuclear
force is
!2
Ã
kT
−47
2
.
(10.15)
σw ∼ 10 m
1 MeV
(Compare this to the Thomson cross-section for the interaction of electrons
via the electromagnetic force: σe = 6.65 × 10−29 m2 .) In the radiationdominated universe, the temperature falls at the rate T ∝ a(t)−1 ∝ t−1/2 , and
thus the cross-sections for weak interactions diminish at the rate σw ∝ t−1 .
The number density of neutrinos falls at the rate nν ∝ a(t)−3 ∝ t−3/2 , and
hence the rate Γ with which neutrons and protons interact with neutrinos
via the weak force falls rapidly:
Γ = nν cσw ∝ t−5/2 .
(10.16)
Meanwhile, the Hubble parameter is only decreasing at the rate H ∝ t−1 .
When Γ ≈ H, the neutrinos decouple from the neutrons and protons, and
the ratio of neutrons to protons is “frozen” (at least until the neutrons start
to decay, at times t ∼ τn ). An exact calculation of the temperature Tfreeze at
216
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
which Γ = H requires a knowledge of the exact cross-section of the proton
and neutron for weak interactions. Using the best available laboratory information, the “freezeout temperature” turns out to be kTfreeze = 0.8 MeV,
or Tfreeze = 9 × 109 K. The universe reaches this temperature when its age is
tfreeze ∼ 1 s. The neutron-to-proton ratio, once the temperature drops below
Tfreeze , is frozen at the value
Qn
nn
= exp −
np
kTfreeze
µ
¶
1.29 MeV
≈ exp −
0.8 MeV
µ
¶
≈ 0.2
(10.17)
At times tfreeze < t ¿ τn , there was one neutron for every five protons in the
universe.
It is the scarcity of neutrons relative to protons that explains why Big
Bang Nucleosynthesis was so incomplete, leaving three-fourths of the baryons
in the form of unfused protons. A neutron will fuse with a proton much more
readily than a proton will fuse with another proton. When two protons fuse
to form a deuterium nucleus, a positron must be emitted (to conserve charge);
this means that an electron neutrino must also be emitted (to conserve electron quantum number). The proton-proton fusion reaction can be written
as
p + p → D + e + + νe .
(10.18)
The involvement of a neutrino in this reaction tells us that it involves the
weak nuclear force, and thus has a minuscule cross-section, of order σw . By
contrast, the neutron-proton fusion reaction is
p+n*
)D+γ .
(10.19)
No neutrinos are involved; this is a strong interaction (one involving the
strong nuclear force). The cross-section for interactions involving the strong
nuclear force are much larger than for those involving the weak nuclear force.
The rate of proton-proton fusion is much slower than the rate of neutronproton fusion, for two reasons. First, the cross-section for proton-proton
fusion, since it is a weak interaction, is minuscule compared to the crosssection for neutron-proton fusion. Second, since protons are all positively
charged, they must surmount the Coulomb barrier between them in order to
fuse.
It’s possible, of course, to coax two protons into fusing with each other.
It’s happening in the Sun, for instance, even as you read this sentence. However, fusion in the Sun is a very slow process. If you pick out any particular
10.2. NEUTRONS AND PROTONS
217
proton in the Sun’s core, it has only one chance in ten billion of undergoing fusion during the next year. Only exceptionally fast protons have any
chance of undergoing fusion, and even those high-speed protons have only a
tiny probability of quantum tunneling through the Coulomb barrier of another proton and fusing with it. The core of the Sun, though, is a stable
environment; it’s in hydrostatic equilibrium, and its temperature and density change only slowly with time. In the early universe, by strong contrast,
the temperature drops as T ∝ t−1/2 and the density of baryons drops as
nbary ∝ t−3/2 . Big Bang Nucleosynthesis is a race against time. After less
than an hour, the temperature and density have dropped too low for fusion
to occur.
For the sake of completeness, I should also note that the rate of neutronneutron fusion in the early universe is negligibly small compared to the rate
of neutron-proton fusion. The reaction governing neutron-neutron fusion is
n + n → D + e− + ν̄e .
(10.20)
Again, the presence of a neutrino (an electron anti-neutrino, in this case) tells
us this is an interaction involving the weak nuclear force. Thus, although
their is no Coulomb barrier between neutrons, the neutron-neutron fusion
rate is tiny. In part, this is because of the scarcity of neutrons relative
to protons, but mainly it is because of the small cross-section for neutronneutron fusion.
Given the alacrity of neutron-proton fusion when compared to the leisurely
rate of proton-proton and neutron-neutron fusion, we can state, as a lowest
order approximation, that BBN proceeds until every free neutron is bonded
into an atomic nucleus, with the leftover protons remaining solitary. In this
approximation, we can compute the maximum possible value of Yp , the fraction of the baryon mass in the form of 4 He. To compute the maximum possible value of Yp , suppose that every neutron present after the proton-neutron
freezeout is incorporated into a 4 He nucleus. Given a neutron-to-proton ratio
of nn /np = 1/5, we can consider a representative group of 2 neutrons and 10
protons. The 2 neutrons can fuse with 2 of the protons to form a single 4 He
nucleus. The remaining 8 protons, though, will remain unfused. The mass
fraction of 4 He will then be
4
1
Ymax =
= .
(10.21)
12
3
More generally, if f ≡ nn /np , with 0 ≤ f ≤ 1, then the maximum possible
value of Yp is Ymax = 2f /(1 + f ).
218
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
If the observed value of Yp = 0.24 were greater than the predicted Ymax ,
that would be a cause for worry; it might mean, for example, that we didn’t
really understand the process of proton-neutron freezeout. However, the fact
that the observed value of Yp is less than Ymax is not worrisome; there are
various factors which act to reduce the actual value of Yp below its theoretical maximum. First, if nucleosynthesis didn’t take place immediately after
freezeout at t ≈ 1 s, then the spontaneous decay of neutrons would inevitably
lower the neutron-to-proton ratio, and thus reduce the amount of 4 He produced. Next, if some neutrons escape fusion altogether, or end in nuclei
lighter than 4 He (such as D or 3 He), they will not contribute to Yp . Finally,
if nucleosynthesis goes on long enough to produce nuclei heavier than 4 He,
that too will reduce Yp .
In order to compute Yp accurately, as well as the abundances of other
isotopes, it will be necessary to consider the process of nuclear fusion in
more detail. Fortunately, much of the statistical mechanics we will need is
just a rehash of what we used when studying recombination.
10.3
Deuterium synthesis
Let’s move on to the next stage of Big Bang Nucleosynthesis, just after
proton-neutron freezeout is complete. The time is t ≈ 2 s. The neutron-toproton ratio is nn /np = 0.2. The neutrinos, which ceased to interact with
electrons about the same time they stopped interacting with neutrons and
protons, are now decoupled from the rest of the universe. The photons,
however, are still strongly coupled to the protons and neutrons. Big Bang
Nucleosynthesis takes place through a series of two-body reactions, building
heavier nuclei step by step. The essential first step in BBN is the fusion of a
proton and a neutron to form a deuterium nucleus:
p+n*
) D+γ .
(10.22)
When a proton and a neutron fuse, the energy released (and carried away by
a gamma ray) is the binding energy of a deuterium nucleus:
BD = (mn + mp − mD )c2 = 2.22 MeV .
(10.23)
Conversely, a photon with energy ≥ BD can photodissociate a deuterium
nucleus into its component proton and neutron. The reaction shown in equation (10.22) should have a haunting familiarity if you’ve just read Chapter 9;
219
10.3. DEUTERIUM SYNTHESIS
it has the same structural form as the reaction governing the recombination
of hydrogen:
(10.24)
p + e− *
)H +γ .
A comparison of equation (10.22) with equation (10.24) shows that in each
case, two particles become bound together to form a composite object, with
the excess energy carried away by a photon. In the case of nucleosynthesis,
a proton and neutron are bonded together by the strong nuclear force to
form a deuterium nucleus, with a gamma-ray photon being emitted. In the
case of photoionization, a proton and electron are bonded together by the
electromagnetic force to form a neutral hydrogen atom, with an ultraviolet
photon being emitted. A major difference between nucleosynthesis and recombination, of course, is between the energy scales involved. The binding
energy of deuterium, BD = 2,200,000 eV, is 160,000 times greater than the
ionization energy of hydrogen, Q = 13.6 eV.3
Despite the difference in energy scales, many of the equations used to
analyze recombination can be re-used to analyze deuterium nucleosynthesis.
Around the time of recombination, for instance, the relative numbers of free
protons, free electrons, and neutral hydrogen atoms is given by the Saha
equation,
!−3/2
Ã
µ
¶
Q
nH
me kT
exp
=
,
(10.25)
np ne
kT
2πh̄2
which tells us that neutral hydrogen is favored in the limit kT → 0, and
that ionized hydrogen is favored in the limit kT → ∞. Around the time
of deuterium synthesis, the relative numbers of free protons, free neutrons,
and deuterium nuclei is given by an equation directly analogous to equation (9.22):
gD
nD
=
np nn
gp gn
Ã
mD
mp mn
!3/2 Ã
kT
2πh̄2
!−3/2
[mp + mn − mD ]c2
exp
kT
Ã
!
. (10.26)
From equation (10.23), we can make the substitution [mp +mn −mD ]c2 = BD .
The statistical weight factor of the deuterium nucleus is gD = 3, in comparison to gp = gn = 2 for a proton or neutron. To acceptable accuracy, we
may write mp = mn = mD /2. These substitutions yield the nucleosynthetic
3
As the makers of bombs have long known, you can release much more energy by fusing
atomic nuclei than by simply shuffling electrons around.
220
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
equivalent of the Saha equation,
Ã
mn kT
nD
=6
np nn
πh̄2
!−3/2
BD
exp
kT
µ
¶
,
(10.27)
which tells us that deuterium is favored in the limit kT → 0, and that free
protons and neutrons are favored in the limit kT → ∞.
To define a precise temperature Tnuc at which the nucleosynthesis of deuterium takes place, we need to define what we mean by “the nucleosynthesis
of deuterium”. Just as recombination is a process which takes a finite length
of time, so is nucleosynthesis. It is useful, though, to define Tnuc as the temperature at which nD /nn = 1; that is, the temperature at which half the free
neutrons have been fused into deuterium nuclei. As long as equation (10.27)
holds true, the deuterium-to-neutron ratio can be written as
Ã
nD
mn kT
= 6np
nn
πh̄2
!−3/2
BD
exp
kT
µ
¶
.
(10.28)
We can write the deuterium-to-neutron ratio as a function of T and the
baryon-to-photon ratio η if we make some simplifying assumptions. Even
today, we know that ∼ 75% of all the baryons in the universe are in the
form of unbound protons. Before the start of deuterium synthesis, 5 out of 6
baryons (or ∼ 83%) were in the form of unbound protons. Thus, if we don’t
want to be fanatical about accuracy, we can write
np ≈ 0.8nbary

Ã
kT
= 0.8ηnγ = 0.8η 0.243
h̄c
!3 
 .
(10.29)
Substituting equation (10.29) into equation (10.28), we find that the deuteriumto-proton ratio is a relatively simple function of temperature:
Ã
nD
kT
≈ 6.5η
nn
mn c2
!3/2
BD
exp
kT
µ
¶
.
(10.30)
This function is plotted in Figure 10.3, assuming a baryon-to-photon ratio of
η = 5.5 × 10−10 . The temperature Tnuc of deuterium nucleosynthesis can be
found by solving the equation
Ã
kTnuc
1 ≈ 6.5η
mn c2
!3/2
BD
exp
kTnuc
µ
¶
.
(10.31)
221
10.3. DEUTERIUM SYNTHESIS
10000
1000
100
nD/nn
10
1
.1
.01
.001
.0001
.9
.8
.7
9
T (10 K)
.6
Figure 10.3: The deuterium-to-neutron ratio during the epoch of deuterium
synthesis. The nucleosynthetic equivalent of the Saha equation (equation 10.27) is assumed to hold true.
222
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
With mn c2 = 939.6 MeV, BD = 2.22 MeV, and η = 5.5 × 10−10 , the temperature of deuterium synthesis is kTnuc ≈ 0.066 MeV, corresponding to
Tnuc ≈ 7.6 × 108 K. The temperature drops to this value when the age
of the universe is tnuc ≈ 200 s.
Note that the time delay until the start of nucleosynthesis, tnuc ≈ 200 s,
is not negligible compared to the decay time of the neutron, τn = 890 s. By
the time nucleosynthesis actually gets underway, neutron decay has slightly
decreased the neutron-to-proton ratio from nn /np = 1/5 to
nn
exp(−200/890)
0.8
≈
≈
≈ 0.15 .
np
5 + [1 − exp(−200/890)]
5.2
(10.32)
This in turn lowers the maximum possible 4 He mass fraction from Ymax ≈ 0.33
to Ymax ≈ 0.27.
10.4
Beyond deuterium
The deuterium-to-neutron ratio nD /nn does not remain indefinitely at the
equilibrium value given by equation (10.30). Once a significant amount of
deuterium forms, there are many possible nuclear reactions available. For
instance, a deuterium nucleus can fuse with a proton to form 3 He:
D+p*
) 3 He + γ .
(10.33)
Alternatively, it can fuse with a neutron to form 3 H, also known as “tritium”:
D+n*
) 3H + γ .
(10.34)
Tritium is unstable; it spontaneously decays to 3 He, emitting an electron and
an electron anti-neutrino in the process. However, the decay time of tritium is
approximately 18 years; during the brief time that Big Bang Nucleosynthesis
lasts, tritium can be regarded as effectively stable.
Deuterium nuclei can also fuse with each other to form 4 He:
D+D*
) 4 He + γ .
(10.35)
However, it is more likely that the interaction of two deuterium nuclei will
end in the formation of a tritium nucleus (with the emission of a proton),
D+D*
) 3H + p ,
(10.36)
223
10.4. BEYOND DEUTERIUM
or the formation of a 3 He nucleus (with the emission of a neutron),
D+D*
) 3 He + n .
(10.37)
There is never a large amount of 3 H or 3 He present during the time of nucleosynthesis. Soon after they are formed, they are converted to 4 He by
reactions such as
3
H+p
3
He + n
3
H+D
3
He + D
*
)
*
)
*
)
*
)
4
He + γ
He + γ
4
He + n
4
He + p .
4
(10.38)
None of the post-deuterium reactions outlined in equations (10.33) through
(10.38) involves neutrinos; they are all involve the strong nuclear force, and
have large cross-sections and fast reaction rates. Thus, once nucleosynthesis
begins, D, 3 H, and 3 He are all efficiently converted to 4 He.
Once 4 He is reached, however, the orderly march of nucleosynthesis to
heavier and heavier nuclei reaches a roadblock. For such a light nucleus, 4 He
is exceptionally tightly bound, as illustrated in Figure 10.1. By contrast,
there are no stable nuclei with A = 5. If you try to fuse a proton or neutron
to 4 He, it won’t work; 5 He and 5 Li are not stable nuclei. Thus, 4 He is resistant
to fusion with protons and neutrons. Small amounts of 6 Li and 7 Li, the two
stable isotopes of lithium, are made by reactions such as
4
He + D *
) 6 Li + γ
(10.39)
He + 3 H *
) 7 Li + γ .
(10.40)
and
4
In addition, small amounts of 7 Be are made by reactions such as
4
He + 3 He *
) 7 Be + γ .
(10.41)
The synthesis of nuclei with A > 7 is hindered by the absence of stable nuclei
with A = 8. For instance, if 8 Be is made by the reaction
4
He + 4 He → 8 Be ,
(10.42)
the 8 Be nucleus falls back apart into a pair of 4 He nuclei with a decay time
of only τ = 3 × 10−16 s.
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CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
Figure 10.4: Mass fraction of nuclei as a function of time during the epoch
of nucleosynthesis. A baryon-to-photon ratio of η = 5.1 × 10−10 is assumed
(from Tytler et al. (2000) Physica Scripta, T85, 12).
The bottom line is that once deuterium begins to be formed, fusion up
to the tightly bound 4 He nucleus proceeds very rapidly. Fusion of heavier
nuclei occurs much less rapidly. The precise yields of the different isotopes
involved in BBN are customarily calculated using a fairly complex computer
code. The complexity is necessary because of the large number of possible
reactions which can occur once deuterium has been formed, all of which
have temperature-dependent cross-sections. Thus, there’s a good deal of
bookkeeping involved. The results of a typical BBN code, which follows
the mass fraction of different isotopes as the universe expands and cools, is
shown in Figure 10.4. Initially, at T À 109 K, almost all the baryonic matter
is in the form of free protons and neutrons. As the deuterium density climbs
upward, however, the point is eventually reached where significant amounts
of 3 H, 3 He, and 4 He are formed. By the time the temperature has dropped
to T ∼ 4 × 108 K, at t ∼ 10 min, Big Bang Nucleosynthesis is essentially
over. Nearly all the baryons are in the form of free protons or 4 He nuclei.
10.4. BEYOND DEUTERIUM
225
The small residue of free neutrons decays into protons. Small amounts of D,
3
H, and 3 He are left over, a tribute to the incomplete nature of Big Bang
Nucleosynthesis. (The 3 H later decays to 3 He.) Very small amounts of 6 Li,
7
Li, and 7 Be are made. (The 7 Be is later converted to 7 Li by electron capture:
7
Be + e− → 7 Li + νe .)
The yields of D, 3 He, 4 He, 6 Li, and 7 Li depend on various physical parameters. Most importantly, they depend on the baryon-to-photon ratio
η. A high baryon-to-photon ratio increases the temperature Tnuc at which
deuterium synthesis occurs, and hence gives an earlier start to Big Bang
Nucleosynthesis. Since BBN is a race against the clock as the density and
temperature of the universe drop, getting an earlier start means that nucleosynthesis is more efficient at producing 4 He, leaving less D and 3 He as
“leftovers”. A plot of the mass fraction of various elements produced by Big
Bang Nucleosynthesis is shown in Figure 10.5. Note that larger values of η
produce larger values for Yp (the 4 He mass fraction) and smaller values for the
deuterium density, as explained above. The dependence of the 7 Li density
on η is more complicated. Within the range of η plotted in Figure 10.5, the
direct production of 7 Li by the fusion of 4 He and 3 H is a decreasing function
of η, while the indirect production of 7 Li by 7 Be electron capture is an increasing function of η. The net result is a minimum in the predicted density
of 7 Li at η ≈ 3 × 10−10 .
Broadly speaking, we know immediately that the baryon-to-photon ratio
can’t be as small as η ∼ 10−12 . If it were, BBN would be extremely inefficient, and we would expect only tiny amounts of helium to be produced
(Yp < 0.01). Conversely, we know that the baryon-to-photon ratio can’t be
as large as η ∼ 10−7 . If it were, nucleosynthesis would have taken place very
early (before neutrons had a chance to decay), the universe would be essentially deuterium-free, and Yp would be near its maximum permissible value
of Ymax ≈ 0.33. Pinning down the value of η more accurately requires making
accurate observations of the primordial densities of the light elements; that
is, the densities before nucleosynthesis in stars started to alter the chemical
composition of the universe. In determining the value of η, it is most useful
to determine the primordial abundance of deuterium. This is because the
deuterium abundance is strongly dependent on η in the range of interest.
Thus, determining the deuterium abundance with only modest accuracy will
enable us to determine η fairly well. By contrast, the primordial helium fraction, Yp , has only a weak dependence on η for the range of interest, as shown
in Figure 10.5. Thus, determining η with a fair degree of accuracy would
226
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
Figure 10.5: The mass fraction of 4 He, and the number densities of D, D+3 He,
and 7 Li expressed as a fraction of the H number density. The width of
each line represents the 95% confidence interval in the density (from Burles,
Nollett, and Turner (2001) ApJ, 552, L1).
10.4. BEYOND DEUTERIUM
227
require measuring Yp with fanatic precision.
A major problem in determining the primordial deuterium abundance
is that deuterium is very easily destroyed in stars. When an interstellar
gas cloud collapses gravitationally to form a star, the first fusion reactions
that occur involve the fusion of deuterium into helium. Thus, the abundance of deuterium in the universe tends to decrease with time.4 Deuterium
abundances are customarily given as the ratio of the number of deuterium
atoms to the number of hydrogen atoms (D/H). For instance, in the local
interstellar medium within our Galaxy, astronomers find an average value
D/H ≈ 1.6 × 10−5 ; that is, there’s one deuterium atom for every 60,000 ordinary hydrogen atoms. However, the Sun and the interstellar medium are
contaminated with material that has been cycled through stellar interiors.
Thus, we expect the primordial deuterium-to-hydrogen value to have been
(D/H)p > 1.6 × 10−5 .
Currently, the most promising way to find the primordial value of D/H is
to look at the spectra of distant quasars. We don’t care about the deuterium
within the quasar itself; instead, we just want to use the quasar as a flashlight
to illuminate the intergalactic gas clouds which lie between it and us. If an
intergalactic gas cloud contains no detectable stars, and has very low levels of
elements heavier than lithium, we can hope that its D/H value is close to the
primordial value, and hasn’t been driven downward by the effects of fusion
within stars. Neutral hydrogen atoms within these intergalactic clouds will
absorb photons whose energy corresponds to the Lyman-α transition; that
is, the transition of the atom’s electron from the ground state (n = 1) to
the next higher energy level (n = 2). In an ordinary hydrogen atom (1 H),
the Lyman-α transition corresponds to a wavelength λH = 121.57 nm. In a
deuterium atom, the greater mass of the nucleus causes a small isotopic shift
in the electron’s energy levels. As a consequence, the Lyman-α transition
in deuterium corresponds to a slightly shorter wavelength, λD = 121.54 nm.
When we look at light from a quasar which has passed through an intergalactic cloud at redshift zcl , we will see a strong absorption line at λH (1 + zcl ),
due to absorption from ordinary hydrogen, and a much weaker absorption
line at λD (1 + zcl ), due to absorption from deuterium. Detailed studies of
the strength of the absorption lines in the spectra of different quasars give
results which are consistent with the ratio (D/H) = (3.0 ± 0.4) × 10 −5 . This
4
There are no mechanisms known that will create deuterium in significant amounts
after Big Bang Nucleosynthesis is complete.
228
CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
translates into a baryon-to-photon ratio of η = (5.5 ± 0.5) × 10−10 .
The value of η can be converted into a value for the current baryon density
by the relation
nbary,0 = ηnγ,0 = 0.23 ± 0.02 m−3 .
(10.43)
Since most of the baryons are protons, we can write, to acceptable accuracy,
εbary,0 = (mp c2 )nbary,0 = 210 ± 20 MeV m−3 .
(10.44)
The current density parameter in baryons is thus
Ωbary,0 =
εbary,0
210 ± 20 MeV m−3
= 0.04 ± 0.01 .
=
εc,0
5200 ± 1000 MeV m−3
(10.45)
Note that the largest source of uncertainty in the value of Ωbary,0 is not
the uncertainty in the baryon density, but the uncertainty in the critical
density (which in turn results from the fact that H0 is not particularly well
determined).
10.5
Baryon – antibaryon asymmetry
The results of Big Bang Nucleosynthesis tell us what the universe was like
when it was relatively hot (Tnuc ≈ 7 × 108 K) and dense:
4
εnuc ≈ αTnuc
≈ 1033 MeV m−3 .
(10.46)
This energy density corresponds to a mass density of εnuc /c2 ≈ 2000 kg m−3 ,
or about twice the density of water. Remember, though, that the energy
density at the time of BBN was almost entirely in the form of radiation. The
mass density of baryons at the time of BBN was
ρbary (tnuc ) = Ωbary,0 ρc,0
µ
Tnuc
T0
≈ (0.04)(9.2 × 10
¶3
−27
(10.47)
7 × 108
kg m )
2.725
−3
Ã
!3
≈ 0.007 kg m−3 .
A density of several grams per cubic meter is not outlandishly high, by everyday standards; it’s equal to the density of the Earth’s stratosphere. A
mean photon energy of 2.7kTnuc ≈ 0.2 MeV is not outlandishly high, by everyday standards; you are bombarded with photons of about that energy
10.5. BARYON – ANTIBARYON ASYMMETRY
229
when you have your teeth X-rayed at the dentist. The physics of Big Bang
Nucleosynthesis is well understood.
Some of the initial conditions for Big Bang Nucleosynthesis, however, are
rather puzzling. The baryon-to-photon ratio, η ≈ 5.5 × 10−10 , is a remarkably small number; the universe seems to have a strong preference for photons
over baryons. It’s also worthy of remark that the universe seems to have a
strong preference for baryons over antibaryons. The laws of physics demand
the presence of antiprotons (p̄), containing two “anti-up” quarks and one
“anti-down” quark apiece, as well as antineutrons (n̄), containing one “antiup” quark and two “anti-down” quarks apiece.5 In practice, though, it is
found that the universe has an extremely large excess of protons and neutrons over antiprotons and antineutrons (and hence an excess of quarks over
antiquarks). At the time of Big Bang Nucleosynthesis, the number density of
antibaryons (n̄ and p̄) was tiny compared to the number density of baryons,
which in turn was tiny compared to the number density of photons. This
imbalance, nantibary ¿ nbary ¿ nγ , has its origin in the physics of the very
early universe.
When the temperature of the early universe was greater than kT ≈
150 MeV, the quarks which it contained were not confined within baryons
and other particles, as they are today, but formed a sea of free quarks (sometimes referred to by the oddly culinary name of “quark soup”). During the
first few microseconds of the universe, when the quark soup was piping hot,
quarks and antiquarks were constantly being created by pair production and
destroyed by mutual annihilation:
γ+γ *
) q + q̄ ,
(10.48)
where q and q̄ could represent, for instance, an “up” quark and an “anti-up”
quark, or a “down” quark and an “anti-down” quark. During this period
of quark pair production, the numbers of “up” quarks, “anti-up” quarks,
“down” quarks, “anti-down” quarks, and photons were nearly equal to each
other. However, suppose there were a very tiny asymmetry between quarks
and antiquarks, such that
δq ≡
5
nq − nq̄
¿1.
nq + nq̄
(10.49)
Note that an “anti-up” quark is not the same as a “down” quark; nor is “anti-down”
equivalent to “up”.
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CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
As the universe expanded and the quark soup cooled, quark-antiquark pairs
would no longer be produced. The existing antiquarks would then annihilate with the quarks. However, because of the small excess of quarks over
antiquarks, there would be a residue of quarks with number density
nq
∼ δq .
(10.50)
nγ
Thus, if there were 1,000,000,000 quarks for every 999,999,997 antiquarks in
the early universe, three lucky quarks in a billion would be left over after the
others encountered antiquarks and were annihilated. The leftover quarks,
however, would be surrounded by roughly 2 billion photons, the product
of the annihilations. After the three quarks were bound together into a
baryon at kT ≈ 150 MeV, the resulting baryon-to-photon ratio would be
η ∼ 5 × 10−10 .
Thus, the very strong asymmetry between baryons and antibaryons today
and the large number of photons per baryon are both products of a tiny
asymmetry between quarks and antiquarks in the early universe. The exact
origin of the quark-antiquark asymmetry in the early universe is still not
exactly known. The physicist Andrei Sakharov, as far back as the year 1967,
was the first to outline the necessary physical conditions for producing a
small asymmetry; however, the precise mechanism by which the quarks first
developed their few-parts-per-billion advantage over antiquarks still remains
to be found.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Bernstein (1995), ch. 4, 5, 6: Puts nucleosynthesis in the larger context
of particle physics and thermodynamics.
Weinberg (1993) The First Three Minutes: Gives some of the historical background to the development of Big Bang Nucleosynthesis.
Problems
(10.1) Suppose the neutron decay time were τn = 89 s instead of τn =
890 s, with all other physical parameters unchanged. Estimate Ymax ,
10.5. BARYON – ANTIBARYON ASYMMETRY
231
the maximum possible mass fraction in 4 He, assuming that all available
neutrons are incorporated into 4 He nuclei.
(10.2) Suppose the difference in rest energy of the neutron and proton were
Bn = (mn − mp )c2 = 0.129 MeV instead of Bn = 1.29 MeV, with all
other physical parameters unchanged. Estimate Ymax , the maximum
possible mass fraction in 4 He, assuming that all available neutrons are
incorporated into 4 He nuclei.
(10.3) A fascinating bit of cosmological history is that of George Gamow’s
prediction of the Cosmic Microwave Background in 1948. (Unfortunately, his prediction was premature; by the time the CMB was actually
discovered in the 1960’s, his prediction had fallen into obscurity.) Let’s
see if you can reproduce Gamow’s line of argument. Gamow knew that
nucleosynthesis must have taken place at a temperature Tnuc ≈ 109 K,
and that the age of the universe is currently t0 ≈ 10 Gyr.
Assume that the universe is flat and contains only radiation. With these
assumptions, what was the energy density ε at the time of nucleosynthesis? What was the Hubble parameter H at the time of nucleosynthesis? What was the time tnuc at which nucleosynthesis took place?
What is the current temperature T0 of the radiation filling the universe today? If the universe switched from being radiation-dominated
to being matter-dominated at a redshift zrm > 0, will this increase or
decrease T0 for fixed values of Tnuc and t0 ? Explain your answer.
(10.4) The total luminosity of the stars in our Galaxy is L ≈ 1.4 × 1010 L¯ .
Suppose that the luminosity of our Galaxy has been constant for the
past 10 Gyr. How much energy has our Galaxy emitted in the form of
starlight during that time? Most stars are powered by the fusion of H
into 4 He, with the release of 28.4 MeV for every helium nucleus formed.
How many helium nuclei have been created within stars in our Galaxy
over the course of the past 10 Gyr, assuming that the fusion of H into
4
He is the only significant energy source? If the baryonic mass of our
Galaxy is M ≈ 1011 M¯ , by what amount has the helium fraction Y of
our Galaxy been increased over its primordial value Y4 = 0.24?
(10.5) In section 10.2, it is asserted that the maximum possible value of the
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CHAPTER 10. NUCLEOSYNTHESIS & THE EARLY UNIVERSE
primordial helium fraction is
Ymax =
2f
,
1+f
(10.51)
where f = nn /np ≤ 1 is the neutron-to-proton ratio at the time of
nucleosynthesis. Prove that this assertion is true.
(10.6) The typical energy of a neutrino in the Cosmic Neutrino Background,
as pointed out in Chapter 5, is Eν ∼ kTν ∼ 5 × 10−4 eV. What
is the approximate interaction cross-section σw for one of these cosmic neutrinos? Suppose you had a large lump of 56 Fe (with density
ρ = 7900 kg m−3 ). What is the number density of protons, neutrons,
and electrons within the lump of iron? How far, on average, would
a cosmic neutrino travel through the iron before interacting with a
proton, neutron, or electron? (Assume that the cross-section for interaction is simply σw , regardless of the type of particle the neutrino
interacts with.)
Chapter 11
Inflation & the Very Early
Universe
The observed properties of galaxies, quasars, and supernovae at relatively
small redshift (z < 6) tell us about the universe at times t > 1 Gyr. The
properties of the Cosmic Microwave Background tell us about the universe
at the time of last scattering (zls ≈ 1100, tls ≈ 0.35 Myr). The abundances
of light elements such as deuterium and helium tell us about the universe at
the time of Big Bang Nucleosynthesis (znuc ≈ 3 × 108 , tnuc ≈ 3 min). In fact,
the observation that primordial gas clouds are roughly one-fourth helium by
mass, rather than being all helium or all hydrogen, tells us that we have
a fair understanding of what was happening at the time of neutron-proton
freezeout (zfreeze ≈ 4 × 109 , tfreeze ≈ 1 s).
So far, my description of the Hot Big Bang scenario has been a triumphal
progress, with only minor details (such as the exact determination of H0 ,
Ωm,0 , and ΩΛ,0 ) remaining to be worked out. Whenever a conquering general made a triumphal progress into ancient Rome, a slave stood behind him,
whispering in his ear, “Remember, you are mortal.” Just as every triumphant
general must be reminded of his flawed, imperfect nature, lest he become insufferably arrogant, so every triumphant theory should be carefully inspected
for flaws and imperfections. The standard Hot Big Bang scenario, in which
the early universe was radiation-dominated, has three underlying problems,
called the flatness problem, the horizon problem, and the monopole problem.
The flatness problem can be summarized by the statement, “The universe
is nearly flat today, and was even flatter in the past.” The horizon problem
can be summarized by the statement, “The universe is nearly isotropic and
233
234
CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
homogeneous today, and was even more so in the past.” The monopole problem can be summarized by the statement, “The universe is apparently free of
magnetic monopoles.” To see why these simple statements pose a problem
to the standard Hot Big Bang scenario, it is necessary to go a little deeper
into the physics of the expanding universe.
11.1
The flatness problem
Let’s start by examining the flatness problem. The spatial curvature of the
universe is related to the density parameter Ω by the Friedmann equation:
1 − Ω(t) = −
κc2
.
R02 a(t)2 H(t)2
(11.1)
(Here, I am using the Friedmann equation in the form given by equation 4.29.)
At the present moment, the density parameter and curvature are linked by
the equation
κc2
1 − Ω0 = − 2 2 .
(11.2)
R0 H 0
The results of the type Ia supernova observations and the measurements of
the CMB anisotropy are consistent with the value (see Figure 9.7)
|1 − Ω0 | ≤ 0.2 .
(11.3)
Why should the value of Ω0 be so close to one today? It could have had,
for instance, the value Ω0 = 10−6 or Ω0 = 106 without violating any laws of
physics. We could, of course, invoke coincidence by saying that the initial
conditions for the universe just happened to produce Ω0 ≈ 1 today. After all,
Ω0 = 0.8 or Ω0 = 1.2 aren’t that close to one. However, when you extrapolate
the value of Ω(t) backward into the past, the closeness of Ω to unity becomes
more and more difficult to dismiss as a coincidence.
By combining equations (11.1) and (11.2), we find the equation that gives
the density parameter as a function of time:
1 − Ω(t) =
H02 (1 − Ω0 )
.
H(t)2 a(t)2
(11.4)
When the universe was dominated by radiation and matter, at times t ¿
tmΛ ≈ 10 Gyr, the Hubble parameter was given by equation (6.35):
Ωr,0 Ωm,0
H(t)2
= 4 + 3 .
2
H0
a
a
(11.5)
11.1. THE FLATNESS PROBLEM
235
Thus, the density parameter evolved at the rate
(1 − Ω0 )a2
.
1 − Ω(t) =
Ωr,0 + aΩm,0
(11.6)
During the period when the universe was dominated by radiation and matter,
the deviation of Ω from one was constantly growing. During the radiationdominated phase,
|1 − Ω|r ∝ a2 ∝ t .
(11.7)
During the later matter-dominated phase,
|1 − Ω|m ∝ a ∝ t2/3 .
(11.8)
Suppose, as the available evidence indicates, that the universe is described by
a model close to the Benchmark Model, with Ωr,0 = 8.4 × 10−5 , Ωm,0 = 0.3 ±
0.1, and ΩΛ,0 = 0.7 ± 0.1. At the present, therefore, the density parameter
falls within the limits |1−Ω0 | ≤ 0.2. At the time of radiation-matter equality,
the density parameter Ωrm was equal to one with an accuracy
|1 − Ωrm | ≤ 2 × 10−4 .
(11.9)
If we extrapolate backward to the time of Big Bang Nucleosynthesis, at
anuc ≈ 3.6 × 10−8 , the deviation of the density parameter Ωnuc from one was
only
|1 − Ωnuc | ≤ 3 × 10−14 .
(11.10)
At the time that deuterium was forming, the density of the universe was equal
to the critical density to an accuracy of one part in 30 trillion. If we push our
extrapolation as far back as we dare, to the Planck time at tP ≈ 5 × 10−44 s,
aP ≈ 2 × 10−32 , we find that the density parameter ΩP was extraordinarily
close to one:
|1 − ΩP | ≤ 1 × 10−60 .
(11.11)
The number 10−60 is, of course, very tiny. To make an analogy, in order to
change the Sun’s mass by one part in 1060 , you would have to add or subtract
two electrons. Our very existence depends on the fanatically close balance
between the actual density and the critical density in the early universe. If,
for instance, the deviation of Ω from one at the time of nucleosynthesis had
been one part in 30 thousand instead of one part in 30 trillion, the universe
would have collapsed in a Big Crunch or expanded to a low-density Big Bore
236
CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
after only a few years. In that case, galaxies, stars, planets, and cosmologists
would not have had time to form.
You might try to dismiss the extreme flatness of the the early universe as
a coincidence, by saying, “ΩP might have had any value, but it just happened
to be 1 ± 10−60 .” However, a coincidence at the level of one part in 1060 is
extremely far-fetched. It would be far more satisfactory if we could find a
physical mechanism for flattening the universe early in its history, instead of
relying on extremely contrived initial conditions at the Planck time.
11.2
The horizon problem
The “flatness problem”, the remarkable closeness of Ω to one in the early
universe, is puzzling. It is accompanied, however, by the “horizon problem”,
which is, if anything, even more puzzling. The “horizon problem” is simply
the statement that the universe is nearly homogeneous and isotropic on very
large scales. Why should we regard this as a problem? So far, we’ve treated
the homogeneity and isotropy of the universe as a blessing rather than a curse.
It’s the homogeneity and isotropy of the universe, after all, which permit us to
describe its curvature by the relatively simple Robertson-Walker metric, and
its expansion by the relatively simple Friedmann equation. If the universe
were inhomogeneous and anisotropic on large scales, it would be much more
difficult to describe mathematically.
The universe, however, is under no obligation to make things simple for
cosmologists. To see why the large scale homogeneity and isotropy of the
universe is so unexpected in the standard Hot Big Bang scenario, consider two
antipodal points on the last scattering surface, as illustrated in Figure 11.1.
The current proper distance to the last scattering surface is
dp (t0 ) = c
Z
t0
tls
dt
.
a(t)
(11.12)
Since the last scattering of the CMB photons occurred a long time ago
(tls ¿ t0 ), the current proper distance to the last scattering surface is
only slightly smaller than the current horizon distance. For the Benchmark Model, the current proper distance to the last scattering surface is
dp (t0 ) = 0.98dhor (t0 ). Thus, two antipodal points on the last scattering surface, separated by 180◦ as seen by an observer on Earth, are currently separated by a proper distance of 1.96dhor (t0 ). Since the two points are farther
237
11.2. THE HORIZON PROBLEM
Figure 11.1: In the standard Hot Big Bang scenario, the current proper
distance to the last scattering surface is 98% of the current horizon distance.
apart than the horizon distance, they are causally disconnected. That is, they
haven’t had time to send messages to each other, and in particular, haven’t
had time to come into thermal equilibrium with each other. Nevertheless,
the two points have the same temperature (once the dipole distortion is subtracted) to within one part in 105 . Why? How can two points that haven’t
had time to swap information be so nearly identical in their properties?
The near-isotropy of the Cosmic Microwave Background is still more remarkable when it is recalled that the temperature fluctuations in the CMB
result from the density and velocity fluctuations that existed at the time of
last scattering. In the standard Hot Big Bang scenario, the universe was
matter-dominated at the time of last scattering, so the horizon distance at
that time can be approximated by the value
dhor (tls ) = 2
c
H(tls )
(11.13)
appropriate to a flat, matter-only universe (see section 5.4). Since the Hubble
distance at the time of last scattering was c/H(tls ) ≈ 0.2 Mpc, the horizon
distance at that time was only dhor (tls ) ≈ 0.4 Mpc. Thus, points more than
238
CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
0.4 megaparsecs apart at the time of last scattering were not in causal contact
in the standard Hot Big Bang scenario. The angular-diameter distance to
the last scattering surface is dA ≈ 13 Mpc, as computed in section 9.4. Thus,
points on the last scattering surface that were separated by a horizon distance
will have an angular separation equal to
θhor =
dhor (tls )
0.4 Mpc
≈
≈ 0.03 rad ≈ 2◦
dA
13 Mpc
(11.14)
as seen from the Earth today. Therefore, points on the last scattering surface
separated by an angle as small as ∼ 2◦ were out of contact with each other at
the time the temperature fluctuations were stamped upon the CMB. Nevertheless, we find that δT /T is as small as 10−5 on scales θ > 2◦ (corresponding
to l < 100 in Figure 9.6).
Why should regions which were out of causal contact with each other
at tls have been so nearly homogeneous in their properties? Invoking coincidence (“The different patches just happened to have the same temperature”)
requires a great stretch of the imagination. The surface of last scattering can
be divided into some 20,000 patches, each two degrees across. In the standard Hot Big Bang scenario, the center of each of these patches was out
of touch with the other patches at the time of last scattering. Now, if you
invite two people to a potluck dinner, and they both bring potato salad,
you can dismiss that as coincidence, even if they had 105 different dishes to
chose from. However, if you invite 20,000 people to a potluck dinner, and
they all bring potato salad, it starts to dawn on you that they must have
been in contact with each other: “Psst...let’s all bring potato salad. Pass it
on.” Similarly, it starts to dawn on you that the different patches of the last
scattering surface, in order to be so nearly equal in temperature, must have
been in contact with each other: “Psst...let’s all be at T = 2.725 K when the
universe is 13.5 gigayears old. Pass it on.”
11.3
The monopole problem
The monopole problem – that is, the apparent lack of magnetic monopoles in
the universe – is not a purely cosmological problem, but one that results from
combining the Hot Big Bang scenario with the particle physics concept of a
Grand Unified Theory. In particle physics, a Grand Unified Theory, or GUT,
is a field theory which attempts to unify the electromagnetic force, the weak
11.3. THE MONOPOLE PROBLEM
239
nuclear force, and the strong nuclear force. Unification of forces has been a
goal of scientists since the 1870s, when James Clerk Maxwell demonstrated
that electricity and magnetism are both manifestations of a single underlying
electromagnetic field. Currently, it is customary to speak of the four fundamental forces of nature: gravitational, electromagnetic, weak, and strong. In
the view of many physicists, though, four forces are three too many; they’ve
spent much time and effort to show that two or more of the “fundamental
forces” are actually different aspects of a single underlying force. About a
century after Maxwell, Steven Weinberg, Abdus Salam, and Sheldon Glashow
successfully devised an electroweak theory. They demonstrated that at particle energies greater than Eew ∼ 1 TeV, the electromagnetic force and the
weak force unite to form a single “electroweak” force. The electroweak energy of Eew ∼ 1 TeV corresponds to a temperature Tew ∼ Eew /k ∼ 1016 K;
the universe had this temperature when its age was tew ∼ 10−12 s. Thus,
when the universe was less than a picosecond old, there were only three fundamental forces: the gravitational, strong, and electroweak force. When the
predictions of the electroweak energy were confirmed experimentally, Weinberg, Salam, and Glashow toted home their Nobel Prizes, and physicists
braced themselves for the next step: unifying the electroweak force with the
strong force.
By extrapolating the known properties of the strong and electroweak
forces to higher particle energies, physicists estimate that at an energy EGUT
of roughly 1012 → 1013 TeV, the strong and electroweak forces should be
unified as a single Grand Unified Force. If the GUT energy is EGUT ∼
1012 TeV, this corresponds to a temperature TGUT ∼ 1028 K and an age for
the universe of tGUT ∼ 10−36 s. The GUT energy is about four orders of
magnitude smaller than the Planck energy, EP ∼ 1016 TeV. Physicists are
searching for a Theory of Everything (TOE) which describes how the Grand
Unified Force and the force of gravity ultimately unite to form a single unified
force at the Planck scale. The different unification energy scales, and the
corresponding temperatures and times in the early universe, are shown in
Figure 11.2.
One of the predictions of Grand Unified Theories is that the universe
underwent a phase transition as the temperature dropped below the GUT
temperature. Generally speaking, phase transitions are associated with a
spontaneous loss of symmetry as the temperature of a system is lowered.
Take, as an example, the phase transition known as “freezing water”. At
temperatures T > 273 K, water is liquid. Individual water molecules are
240
CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
gravity
??
TOE
strong
GUT
ew
E (TeV)
T (K)
t (sec)
16
10
32
10
−43
10
12
10
28
10
−36
10
weak
electromagnetic
1
16
10
−12
10
Figure 11.2: The energy, temperature, and time scales at which the different
force unifications occur.
randomly oriented, and the liquid water thus has rotational symmetry about
any point; in other words, it is isotropic. However, when the temperature
drops below T = 273 K, the water undergoes a phase transition, from liquid to solid, and the rotational symmetry of the water is lost. The water
molecules are locked into a crystalline structure, and the ice no longer has
rotational symmetry about an arbitrary point. In other words, the ice crystal is anisotropic, with preferred directions corresponding to the crystal’s
axes of symmetry.1 In a broadly similar vein, there is a loss of symmetry
when the universe undergoes the GUT phase transition at tGUT ∼ 10−36 s.
At T > TGUT , there was a symmetry between the strong and electroweak
forces. At T < TGUT , the symmetry is spontaneously lost; the strong and
electroweak forces begin to behave quite differently from each other.
In general, phase transitions associated with a loss of symmetry give rise
to flaws known as topological defects. To see how topological defects form,
consider a large tub of water which is cooled below T = 273 K. Usually,
the freezing of the water will start at two or more widely separated nucleation sites. The crystal which forms about any given nucleation site is very
regular, with well-defined axes of symmetry. However, the axes of symme1
Suppose, for instance, that the water freezes in the familiar six-pointed form of a
snowflake. It is now only symmetric with respect to rotations of 60◦ (or integral multiples
of that angle) about the snowflake’s center.
241
11.3. THE MONOPOLE PROBLEM
try of two adjacent ice crystals will be misaligned. At the boundary of two
adjacent crystals, there will be a two-dimensional topological defect, called
a domain wall, where the axes of symmetry fail to line up. Other types of
phase transitions give rise to one-dimensional, or line-like, topological defects (in a cosmological context, these linear defects are known as cosmic
strings). Still other types of phase transitions give rise to zero-dimensional,
or point-like, topological defects. Grand Unified Theories predict that the
GUT phase transition creates point-like topological defects which act as magnetic monopoles. That is, they act as the isolated north pole or south pole
of a magnet. The rest energy of the magnetic monopoles created in the GUT
phase transition is predicted to be mM c2 ∼ EGUT ∼ 1012 TeV. This corresponds to a mass of over a nanogram (comparable to that of a bacterium),
which is a lot of mass for a single particle to be carrying around. At the
time of the GUT phase transition, points further apart than the horizon size
will be out of causal contact with each other. Thus, we expect roughly one
topological defect per horizon volume, due to the mismatch of fields which
are not causally linked. The number density of magnetic monopoles, at the
time of their creation, would be
nM (tGUT ) ∼
1
(2ctGUT )3
∼ 1082 m−3 ,
(11.15)
and their energy density would be
εM (tGUT ) ∼ (mM c2 )nM ∼ 1094 TeV m−3 .
(11.16)
This is a large energy density, but it is smaller by ten orders of magnitude
than the energy density of radiation at the time of the GUT phase transition:
4
εγ (tGUT ) ≈ αTGUT
∼ 10104 TeV m−3 .
(11.17)
Thus, the magnetic monopoles wouldn’t have kept the universe from being
radiation-dominated at the time of the GUT phase transition. However, the
magnetic monopoles, being so massive, would soon have become highly nonrelativistic, with energy density εM ∝ a−3 . The energy density in radiation,
though, was falling off at the rate εγ ∝ a−4 . Thus, the magnetic monopoles
would have dominated the energy density of the universe when the scale
factor had grown by a factor ∼ 1010 ; that is, when the temperature had
fallen to T ∼ 10−10 TGUT ∼ 1018 K, and the age of the universe was only
t ∼ 10−16 s.
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CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
Obviously, the universe is not dominated by magnetic monopoles today.
In fact, there is no strong evidence that they exist at all. Every north
magnetic pole which we can find is paired with a south magnetic pole,
and vice versa. There are no isolated north poles or isolated south poles.
The monopole problem can thus be rephrased as the question, “Where have
all the magnetic monopoles gone?” Now, you can always answer the question, “Where have the monopoles gone?” by saying, “There were never any
monopoles to begin with.” There is not yet a single, definitive Grand Unified Theory, and in some variants on the GUT theme, magnetic monopoles
are not produced. However, the flatness and horizon problems are not so
readily dismissed. When the physicist Alan Guth first proposed the idea of
inflation in 1981, he introduced it as a way of resolving the flatness problem,
the horizon problem, and the monopole problem with a single cosmological
mechanism.
11.4
The inflation solution
What is inflation? In a cosmological context, inflation can most generally be
defined as the hypothesis that there was a period, early in the history of our
universe, when the expansion was accelerating outward; that is, an epoch
when ä > 0. The acceleration equation,
ä
4πG
= − 2 (ε + 3P ) ,
a
3c
(11.18)
tells us that ä > 0 when P < −ε/3. Thus, inflation would have taken place if
the universe were temporarily dominated by a component with equation-ofstate parameter w < −1/3. The usual implementation of inflation states that
the universe was temporarily dominated by a positive cosmological constant
Λi (with w = −1), and thus had an acceleration equation that could be
written in the form
Λi
ä
=
>0.
(11.19)
a
3
In an inflationary phase when the energy density was dominated by a cosmological constant, the Friedmann equation was
µ ¶2
ȧ
a
=
Λi
.
3
(11.20)
11.4. THE INFLATION SOLUTION
243
The Hubble constant Hi during the inflationary phase was thus constant,
with the value Hi = (Λi /3)1/2 , and the scale factor grew exponentially with
time:
a(t) ∝ eHi t .
(11.21)
Too see how a period of exponential growth can resolve the flatness, horizon,
and monopole problems, suppose that the universe had a period of exponential expansion sometime in the midst of its early, radiation-dominated
phase. For simplicity, suppose the exponential growth was switched on instantaneously at a time ti , and lasted until some later time tf , when the
exponential growth was switched off instantaneously, and the universe reverted to its former state of radiation-dominated expansion. In this simple
case, we can write the scale factor as

1/2

 ai (t/ti )
t < ti
a(t) =  ai eHi (t−ti )
ti < t < t f

ai eHi (tf −ti ) (t/tf )1/2 t > tf
(11.22)
Thus, between the time ti when the exponential inflation began and the time
tf when the inflation stopped, the scale factor increased by a factor
a(tf )
= eN ,
a(ti )
(11.23)
where N , the number of e-foldings of inflation, was
N ≡ Hi (tf − ti ) .
(11.24)
If the duration of inflation, tf − ti , was long compared to the Hubble time
during inflation, Hi−1 , then N was large, and the growth in scale factor during
inflation was enormous.
For concreteness, let’s take one possible model for inflation. This model
states that exponential inflation started around the GUT time, ti ≈ tGUT ≈
36 −1
10−36 s, with a Hubble parameter Hi ≈ t−1
GUT ≈ 10 s , and lasted for
N ∼ 100 Hubble times. In this particular model, the growth in scale factor
during inflation was
a(tf )
∼ e100 ∼ 1043 .
(11.25)
a(ti )
Note that the cosmological constant Λi present at the time of inflation was
very large compared to the cosmological constant which seems to be present
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CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
today. Currently, the evidence is consistent with an energy density in Λ of
εΛ,0 ≈ 0.7εc,0 ≈ 0.004 TeV m−3 . To produce exponential expansion with a
Hubble parameter Hi ≈ 1036 s−1 , the cosmological constant during inflation
would have had an energy density (see equation 4.65)
ε Λi =
c2
3c2 2
Λi =
H ∼ 10105 TeV m−3 ,
8πG
8πG i
(11.26)
over 107 orders of magnitude larger.
How does inflation resolve the flatness problem? Equation (11.1), which
gives Ω as a function of time, can be written in the form
|1 − Ω(t)| =
c2
R02 a(t)2 H(t)2
(11.27)
for any universe which is not perfectly flat. If the universe is dominated
by a single component with equation-of-state parameter w 6= −1, then a ∝
t2/(3+3w) , H ∝ t−1 , and
|1 − Ω(t)| ∝ t2(1+3w)/(3+3w) .
(11.28)
Thus, if w < −1/3, the difference between Ω and one decreases with time. If
the universe is expanding exponentially during the inflationary epoch, then
|1 − Ω(t)| ∝ e−2Hi t ,
(11.29)
and the difference between Ω and one decreases exponentially with time. If we
compare the density parameter at the beginning of exponential inflation (t =
ti ) with the density parameter at the end of inflation (t = tf = ti + N/Hi ),
we find
|1 − Ω(tf )| = e−2N |1 − Ω(ti )| .
(11.30)
Suppose that prior to inflation, the universe was actually fairly strongly
curved, with
|1 − Ω(ti )| ∼ 1 .
(11.31)
After a hundred e-foldings of inflation, the deviation of Ω from one would be
|1 − Ω(tf )| ∼ e−2N ∼ e−200 ∼ 10−87 .
(11.32)
Even if the universe at ti wasn’t particularly close to being flat, a hundred efoldings of inflation would flatten it like the proverbial pancake. The current
245
11.4. THE INFLATION SOLUTION
limits on the density parameter, |1−Ω0 | ≤ 0.2, imply that N > 60, if inflation
took place around the GUT time. However, it’s possible that N may have
been very much greater than 60, since the observational data are entirely
consistent with |1 − Ω0 | ¿ 1.
How does inflation resolve the horizon problem?
At any time t, the
horizon distance dhor (t) is given by the relation
dhor (t) = a(t)c
Z
t
0
dt
.
a(t)
(11.33)
Prior to the inflationary period, the universe was radiation-dominated. Thus,
the horizon time at the beginning of inflation was
dhor (ti ) = ai c
Z
ti
0
dt
= 2cti .
ai (t/ti )1/2
(11.34)
The horizon size at the end of inflation was
N
dhor (tf ) = ai e c
ÃZ
ti
0
Z tf
dt
dt
+
1/2
ai (t/ti )
ti ai exp[Hi (t − ti )]
!
.
(11.35)
If N , the number of e-foldings of inflation, is large, then the horizon size at
the end of inflation was
dhor (tf ) = eN c(2ti + Hi−1 ) .
(11.36)
An epoch of exponential inflation causes the horizon size to grow exponentially. If inflation started at ti ≈ 10−36 s, with a Hubble parameter
Hi ≈ 1036 s−1 , and lasted for n ≈ 100 e-foldings, then the horizon size immediately before inflation was
dhor (ti ) = 2cti ≈ 6 × 10−28 m .
(11.37)
The horizon size immediately after inflation was
dhor (tf ) ≈ eN 3cti ≈ 2 × 1016 m ≈ 0.8 pc .
(11.38)
During the brief period of ∼ 10−34 s that inflation lasts in this model, the
horizon size is boosted exponentially from submicroscopic scales to nearly
a parsec. At the end of the inflationary epoch, the horizon size reverts to
growing at a sedate linear rate.
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CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
The net result of inflation is to increase the horizon length in the postinflationary universe by a factor ∼ eN over what it would have been without
inflation. For instance, we found that, in the absence of inflation, the horizon
size at the time of last scattering was dhor (tls ) ≈ 0.4 Mpc. Given a hundred
e-foldings of inflation in the early universe, however, the horizon size at last
scattering would have been ∼ 1043 Mpc, obviously gargantuan enough for the
entire last scattering surface to be in causal contact.
To look at the resolution of the horizon problem from a slightly different
viewpoint, consider the entire universe directly visible to us today; that is,
the region bounded by the surface of last scattering. Currently, the proper
distance to the surface of last scattering is
dp (t0 ) ≈ 1.4 × 104 Mpc .
(11.39)
If inflation ended at tf ∼ 10−34 s, that corresponds to a scale factor af ∼
2 × 10−27 . Thus, immediately after inflation, the portion of the universe
currently visible to us was crammed into a sphere of proper radius
dp (tf ) = af dp (t0 ) ∼ 3 × 10−23 Mpc ∼ 0.9 m .
(11.40)
Immediately after inflation, in this model, all the mass-energy destined to
become the hundreds of billions of galaxies we see today was enclosed within
a sphere only six feet across. But, to quote Al Jolson, you ain’t heard nothin’
yet, folks. If there were N = 100 e-foldings of inflation, then immediately
prior to the inflationary epoch, the currently visible universe was enclosed
within a sphere of proper radius
dp (ti ) = e−N dp (tf ) ∼ 3 × 10−44 m .
(11.41)
Note that this distance is 16 orders of magnitude smaller than the horizon
size immediately prior to inflation (dhor (ti ) ∼ 6×10−28 m.) Thus, the portion
of the universe which we can see today had plenty of time to achieve thermal
uniformity before inflation began.
How does inflation resolve the monopole problem? If magnetic monopoles
were created before or during inflation, then the number density of monopoles
was diluted to any undetectably low level. During a period when the universe
was expanding exponentially (a ∝ eHi t ), the number density of monopoles,
if they were neither created nor destroyed, was decreasing exponentially
(nM ∝ e−3Hi t ). For instance, if inflation started around the GUT time,
11.5. THE PHYSICS OF INFLATION
247
when the number density of magnetic monopoles was nM (tGUT ) ≈ 1082 m−3 ,
then after 100 e-foldings of inflation, the number density would have been
nM (tf ) = e−300 nM (tGUT ) ≈ 5 × 10−49 m−3 ≈ 15 pc−3 . The number density
today, after the additional expansion from a(tf ) ≈ 2 × 10−27 to a0 = 1, would
then be nM (t0 ) ≈ 1 × 10−61 Mpc−3 . The probability of finding even a single
monopole within the last scattering surface would be astronomically small.
11.5
The physics of inflation
Inflation explains some otherwise puzzling aspects of our universe, by flattening it, ensuring its homogeneity over large scales, and driving down the
number density of magnetic monopoles which it contains. However, I have
not yet answered many crucial questions about the inflationary epoch. What
triggers inflation at t = ti , and (just as important) what turns it off at t = tf ?
If inflation reduces the number density of monopoles to undetectably low
levels, why doesn’t it reduce the number density of photons to undetectably
low levels? If inflation is so efficient at flattening the global curvature of
the universe, why doesn’t it also flatten out the local curvature due to fluctuations in the energy density? We know that the universe wasn’t perfectly
homogeneous after inflation, because the Cosmic Microwave Background isn’t
perfectly isotropic.
To answer these questions, we will have to examine, at least in broad
outline, the physics behind inflation. At present, there is not a consensus
among cosmologists about the exact mechanism driving inflation. I will restrict myself to speaking in general terms about one plausible mechanism for
bringing about an inflationary epoch.
Suppose the universe contains a scalar field φ(~r, t) whose value can vary
as a function of position and time. (Some early implementations of inflation
associated the scalar field φ with the Higgs field which mediates interactions
between particles at energies higher than the GUT energy; however, to keep
the discussion general, let’s just call the field φ(~r, t) the inflaton field.) Generally speaking, a scalar field can have an associated potential energy V (φ).2
If φ has units of energy, and its potential V has units of energy density,
2
As a simple illustrative example, suppose that the scalar field φ is the elevation above
sea level at a given point on the Earth’s surface. The associated potential energy, in this
case, is the gravitational potential V = gφ, where g = 9.8 km s−1 .
248
CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
then the energy density of the inflaton field is
εφ =
1 1 2
φ̇ + V (φ)
2 h̄c3
(11.42)
in a region of space where φ is homogeneous. The pressure of the inflaton
field is
1 1 2
Pφ =
φ̇ − V (φ) .
(11.43)
2 h̄c3
If the inflaton field changes only very slowly as a function of time, with
φ̇2 ¿ h̄c3 V (φ) ,
(11.44)
then the inflaton field acts like a cosmological constant, with
εφ ≈ −Pφ ≈ V (φ) .
(11.45)
Thus, an inflaton field can drive exponential inflation if there is a temporary
period when its rate of change φ̇ is small (satisfying equation 11.44), and its
potential V (φ) is large enough to dominate the energy density of the universe.
Under what circumstances are the conditions for inflation (small φ̇ and
large V ) met in the early universe? To determine the value of φ̇, start with
the fluid equation for the energy density of the inflaton field,
ε̇φ + 3H(t)(εφ + Pφ ) = 0 ,
(11.46)
where H(t) = ȧ/a. Substituting from equations (11.42) and (11.43), we find
the equation which governs the rate of change of φ:
φ̈ + 3H(t)φ̇ = −h̄c3
dV
.
dφ
(11.47)
Note that equation (11.47) mimics the equation of motion for a particle which
is being accelerated by a force proportional to −dV /dφ and being impeded by
a frictional force proportional to the particle’s speed. Thus, the expansion
of the universe provides a “Hubble friction” term, 3H φ̇, which slows the
transition of the inflaton field to a value which will minimize the potential
V . Just as a skydiver reaches terminal velocity when the downward force of
gravity is balanced by the upward force of air resistance, so the inflaton field
can reach “terminal velocity” (with φ̈ = 0) when
3H φ̇ = −h̄c3
dV
,
dφ
(11.48)
249
11.5. THE PHYSICS OF INFLATION
or
h̄c3 dV
.
(11.49)
3H dφ
If the inflaton field has reached this terminal velocity, then the requirement
that φ̇2 ¿ h̄c3 V , necessary if the inflaton field is to play the role of a cosmological constant, translates into
φ̇ = −
Ã
dV
dφ
!2
¿
9H 2 V
.
h̄c3
(11.50)
If the universe is undergoing exponential inflation driven by the potential
energy of the inflaton field, this means that the Hubble parameter is
H=
µ
8πGεφ
3c2
¶1/2
=
µ
8πGV
3c2
¶1/2
,
(11.51)
and equation (11.50) becomes
Ã
dV
dφ
!2
¿
24πGV 2
,
h̄c5
(11.52)
which can also be written as
Ã
EP dV
V dφ
!2
¿1,
(11.53)
where EP is the Planck energy. If the slope of the inflaton’s potential is
sufficiently shallow, satisfying equation (11.53), and if the amplitude of the
potential is sufficiently large to dominate the energy density of the universe,
then the inflaton field is capable of giving rise to exponential expansion.
As a concrete example of a potential V (φ) which can give rise to inflation,
consider the potential shown in Figure 11.3. The global minimum in the
potential occurs when the value of the inflaton field is φ = φ0 . Suppose,
however, that the inflaton field starts at φ ≈ 0, where the potential is V (φ) ≈
V0 . If
!2
Ã
V2
dV
¿ 02
(11.54)
dφ
EP
on the “plateau” where V ≈ V0 , then while φ is slowly rolling toward φ0 ,
the inflaton field contributes an energy density εφ ≈ V0 ≈ constant to the
universe.
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CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
V
V0
0
0
φ0
φ
Figure 11.3: A potential which can give rise to an inflationary epoch. The
global minimum in V (or “true vacuum”) is at φ = φ0 . If the scalar field
starts at φ = 0, it is in a “false vacuum” state.
251
11.5. THE PHYSICS OF INFLATION
When an inflaton field has a potential similar to that of Figure 11.3, it
is referred to as being in a metastable false vacuum state when it is near the
maximum at φ = 0. Such a state is not truly stable; if the inflaton field is
nudged from φ = 0 to φ = +dφ, it will continue to slowly roll toward the true
vacuum state at φ = φ0 and V = 0. However, if the plateau is sufficiently
broad as well as sufficiently shallow, it can take many Hubble times for the
inflaton field to roll down to the true vacuum state. Whether the inflaton
field is dynamically significant during its transition from the false vacuum
to the true vacuum depends on the value of V0 . As long as εφ ≈ V0 is tiny
compared to the energy density of radiation, εr ∼ αT 4 , the contribution of
the inflaton field to the Friedmann equation can be ignored. Exponential
inflation, driven by the energy density of the inflaton field, will begin at a
temperature
Ti ≈
or
µ
V0
α
¶1/4
≈ 2 × 1028 K
µ
V0
10105 TeV m−3
V0
kTi ≈ (h̄ c V0 ) ≈ 2 × 10 TeV
105
10 TeV m−3
This corresponds to a time
3 3
ti ≈
Ã
c2
GV0
1/4
!1/2
µ
12
≈ 3 × 10
−36
V0
s
105
10 TeV m−3
µ
¶1/4
¶1/4
¶−1/2
.
(11.55)
.
(11.56)
(11.57)
While the inflaton field is slowly rolling toward the true vacuum state, it
produces exponential expansion, with a Hubble parameter
Hi ≈
µ
8πGV0
3c2
¶1/2
≈ t−1
.
i
(11.58)
The exponential expansion ends as the inflaton field reaches the true vacuum
at φ = φ0 . The duration of inflation thus depends on the exact shape of
the potential V (φ). The number of e-foldings of inflation, for the potential
shown in Figure 11.3, should be
φ0
N ∼ Hi ∼
φ̇
Ã
EP dV
V0 dφ
!−1 Ã
φ0
EP
!
.
(11.59)
Large values of φ0 and V0 (that is, a broad, high plateau) and small values
of dV /dφ (that is, a shallowly sloped plateau) lead to more e-foldings of
inflation.
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CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
After rolling off the plateau in Figure 11.3, the inflaton field φ oscillates
about the minimum at φ0 . The amplitude of these oscillations is damped by
the “Hubble friction” term proportional to H φ̇ in equation (11.47). If the
inflaton field is coupled to any of the other fields in the universe, however, the
oscillations in φ are damped more rapidly, with the energy of the inflaton
field being carried away by photons or other relativistic particles. These
photons reheat the universe after the precipitous drop in temperature caused
by inflation. The energy lost by the inflaton field after its phase transition
from the false vacuum to the true vacuum can be thought of as the latent heat
of that transition. When water freezes, to use a low-energy analogy, it loses
an energy of 3 × 108 J m−3 , which goes to heat its surroundings.3 Similarly,
the transition from false to true vacuum releases an energy V0 which goes to
reheat the universe.
If the scale factor increases by a factor
a(tf )
= eN
a(ti )
(11.60)
during inflation, then the temperature will drop by a factor e−N . If inflation
starts around the GUT time, and lasts for N = 100 e-foldings, then the
temperature drops from a toasty T (ti ) ∼ TGUT ∼ 1028 K to a chilly T (tf ) ∼
e−100 TGUT ∼ 10−15 K. At a temperature of 10−15 K, you’d expect to find
a single photon in a box 25 AU on a side, as compared to the 411 million
photons packed into every cubic meter of space today. Not only is inflation
very effective at driving down the number density of magnetic monopoles, it
is also effective at driving down the number density of every other type of
particle, including photons. The chilly post-inflationary period didn’t last,
though. As the energy density associated with the inflaton field was converted
to relativistic particles such as photons, the temperature of the universe was
restored to its pre-inflationary value Ti .
Inflation successfully explains the flatness, homogeneity, and isotropy of
the universe. It ensures that we live in a universe with a negligibly low density of magnetic monopoles, while the inclusion of reheating ensures that we
don’t live in a universe with a negligibly low density of photons. In some
3
This is why orange growers spray their trees with water when a hard freeze threatens.
The energy released by water as it freezes keeps the delicate leaves warm. (The thin layer
of ice also cuts down on convective and radiative heat loss, but the release of latent heat
is the largest effect.)
11.5. THE PHYSICS OF INFLATION
253
ways, though, inflation seems to be too successful. It makes the universe
homogeneous and isotropic all right, but it makes it too homogeneous and
isotropic. One hundred e-foldings of inflation not only flattens the global curvature of the universe, it also flattens the local curvature due to fluctuations
in the energy density. If energy fluctuations prior to inflation were δε/ε̄ ∼ 1,
a naı̈ve calculation predicts that density fluctuations immediately after 100
e-foldings of inflation would be
δε
∼ e−100 ∼ 10−43 .
ε̄
(11.61)
This is a very close approach to homogeneity. Even allowing for the growth
in amplitude of density fluctuations prior to the time of last scattering, this
would leave the Cosmic Microwave Background much smoother than is actually observed.
Remember, however, the saga of how a submicroscopic patch of the universe (d ∼ 3 × 10−44 m) was inflated to macroscopic size (d ∼ 1 m), before
growing to the size of the currently visible universe. Inflation excels in taking submicroscopic scales and blowing them up to macroscopic scales. On
submicroscopic scales, the vacuum, whether true or false, is full of constantly
changing quantum fluctuations, with virtual particles popping into and out
of existence. On quantum scales, the universe is intrinsically inhomogeneous.
Inflation takes the submicroscopic quantum fluctuations in the inflaton field
and expands them to macroscopic scales. The energy fluctuations that result
are the origin, in the inflationary scenario, of the inhomogeneities in the current universe. We can replace the old proverb, “Great oaks from tiny acorns
grow,” with the yet more amazing proverb, “Great superclusters from tiny
quantum fluctuations grow.”
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Islam (2002), ch. 9: A general overview of inflation, avoiding technical
concepts of particle physics.
Liddle (1999), ch. 11: A brief, clear discussion of how inflation solves the
horizon, flatness, and monopole problems.
254
CHAPTER 11. INFLATION & THE VERY EARLY UNIVERSE
Liddle & Lyth (2000): A thorough, recent review of all aspects of inflationary cosmology.
Problems
(11.1) What upper limit is placed on Ω(tP ) by the requirement that the
universe not end in a Big Crunch between the Planck time, tP ≈ 5 ×
10−44 s, and the start of the inflationary epoch at ti ? Compute the
maximum permissible value of Ω(tP ), first assuming ti ≈ 10−36 s, then
assuming ti ≈ 10−26 s. (Hint: prior to inflation, the Friedmann equation
will be dominated by the radiation term and the curvature term.)
(11.2) Current observational limits on the density of magnetic monopoles
tell us that their density parameter is currently ΩM,0 < 10−6 . If
monopoles formed at the GUT time, with one monopole per horizon of mass mM = mGUT , how many e-foldings of inflation would be
required to drive the current density of monopoles below the bound
ΩM,0 < 10−6 ? Assume that inflation took place immediately after the
formation of monopoles.
(11.3) It has been speculated that the present-day acceleration of the universe is due to the existence of a false vacuum which will eventually decay. Suppose that the energy density of the false vacuum is
εΛ = 0.7εc,0 = 3600 MeV m−3 , and that the current energy density of
matter is εm,0 = 0.3εc,0 = 1600 MeV m−3 . What will be the value of the
Hubble parameter once the false vacuum becomes strongly dominant?
Suppose that the false vacuum is fated to decay instantaneously to radiation at a time tf = 50t0 . (Assume, for simplicity, that the radiation
takes the form of blackbody photons.) To what temperature will the
universe be reheated at t = tf ? What will the energy density of matter
be at t = tf ? At what time will the universe again be dominated by
matter?
Chapter 12
The Formation of Structure
The universe can be approximated as being homogeneous and isotropic only
if we smooth it with a filter ∼ 100 Mpc across. On smaller scales, the universe
contains density fluctuations ranging from subatomic quantum fluctuations
up to the large superclusters and voids, ∼ 50 Mpc across, which characterize the distribution of galaxies in space. If we relax the strict assumption
of homogeneity and isotropy which underlies the Robertson-Walker metric
and the Friedmann equation, we can ask (and, to some extent, answer) the
question, “How do density fluctuations in the universe evolve with time?”
The formation of relatively small objects, such as planets, stars, or even
galaxies, involves some fairly complicated physics. Consider a galaxy, for
instance. As mentioned in Chapter 8, the luminous portions of galaxies are
typically much smaller than the dark halos in which they are embedded. In
the usual scenario for galaxy formation, this is because the baryonic component of a galaxy radiates away energy, in the form of photons, and slides to
the bottom of the potential well defined by the dark matter. The baryonic
gas then fragments to form stars, in a nonlinear magnetohydrodynamical
process.
In this chapter, however, I will be focusing on the formation of structures
larger than galaxies – clusters, superclusters, and voids. Cosmologists use the
term “large scale structure of the universe” to refer to all structures bigger
than individual galaxies. A map of the large scale structure of the universe,
as traced by the positions of galaxies, can be made by measuring the redshifts
of a sample of galaxies and using the Hubble relation, d = (c/H0 )z, to compute their distances from our own Galaxy. For instance, Figure 12.1 shows a
redshift map from the 2dF Galaxy Redshift Survey. By plotting redshift as
255
256
CHAPTER 12. THE FORMATION OF STRUCTURE
Figure 12.1: A redshift map of ∼ 105 galaxies, in a strip ∼ 75◦ long, from
right ascension α ≈ 10 h to α ≈ 15 h, and ∼ 8◦ wide, from declination
δ ≈ −5◦ to δ ≈ 3◦ . (Image courtesy of the 2dF Galaxy Redshift Survey
team.)
257
Figure 12.2: The northeastern United States and southeastern Canada at
night, as seen by a satellite from the Defense Meteorological Satellite Program
(DMSP).
a function of angular position for galaxies in a long, narrow strip of the sky, a
“slice of the universe” can be mapped. In a slice such as that of Figure 12.1,
which reaches to z ≈ 0.3, or dp (t0 ) ≈ 1300 Mpc, the galaxies obviously do
not have a random Poisson distribution. The most prominent structures in
Figure 12.1 are superclusters and voids. Superclusters are objects which are
just in the process of collapsing under their own self-gravity. Superclusters
are typically flattened (roughly planar) or elongated (roughly linear) structures. A supercluster will contain one or more clusters embedded within it;
a cluster is a fully collapsed object which has come to equilibrium (more or
less), and hence obeys the virial theorem, as discussed in section 8.3. In
comparison to the flattened or elongated superclusters, the underdense voids
are roughly spherical in shape. When gazing at the large scale structure of
the universe, as traced by the distribution of galaxies, cosmologists are likely
to call it “bubbly” or “spongy” or “frothy” or “foamy”.
Being able to describe the distribution of galaxies in space doesn’t automatically lead to an understanding of the origin of large scale structure.
Consider, as an analogy, the distribution of luminous objects shown in Figure 12.2. The distribution of illuminated cities on the Earth’s surface is obviously not random. There are “superclusters” of cities, such as the Boswash
supercluster stretching from Boston to Washington. There are “voids” such
as the Appalachian void. However, the influences which determine the exact
258
CHAPTER 12. THE FORMATION OF STRUCTURE
location of cities are often far removed from fundamental physics.1
Fortunately, the distribution of galaxies in space is more closely tied to
fundamental physics than is the distribution of cities on the Earth. The basic
mechanism for growing large structures, such as voids and superclusters, is
gravitational instability. Suppose that at some time in the past, the density
of the universe had slight inhomogeneities. We know, for instance, that such
density fluctuations occurred at the time of last scattering, since they left
their stamp on the Cosmic Microwave Background. When the universe is
matter-dominated, the overdense regions expand less rapidly than the universe as a whole; if their density is sufficiently great, they will collapse and
become gravitationally bound objects such as clusters. The dense clusters
will, in addition, draw matter to themselves from the surrounding underdense
regions. The effect of gravity on density fluctuations is sometimes referred
to as the Matthew Effect: “For whosoever hath, to him shall be given, and
he shall have more abundance; but whosoever hath not, from him shall be
taken away even that he hath” (Matthew 13:12). In less biblical language,
the rich get richer and the poor get poorer.
12.1
Gravitational instability
To put our study of gravitational instability on a more quantitative basis,
consider some component of the universe whose energy density ε(~r, t) is a
function of position as well as time. At a given time t, the spatially averaged
energy density is
1 Z
ε(~r, t)d3 r .
(12.1)
ε̄(t) =
V V
To ensure that we have found the true average, the volume V over which we
are averaging must be large compared to the size of the biggest structure in
the universe. It is useful to define a dimensionless density fluctuation
δ(~r, t) ≡
ε(~r, t) − ε̄(t)
.
ε̄(t)
(12.2)
The value of δ is thus negative in underdense regions and positive in overdense
regions. The minimum possible value of δ is δ = −1, corresponding to ε = 0.
1
Consider, for instance, the complicated politics that went into determining the location
of Washington, DC.
259
12.1. GRAVITATIONAL INSTABILITY
R(t)
ρ = ρ(1+δ)
Figure 12.3: A sphere of radius R(t) expanding or contracting under the
influence of the density fluctuation δ(t).
In principle, there is no upper limit on δ. You, for instance, represent a
region of space where the baryon density has δ ≈ 2 × 1030 .
The study of how large scale structure evolves with time requires knowing
how a small fluctuation in density, with |δ| ¿ 1, grows in amplitude under
the influence of gravity. This problem is most tractable when |δ| remains very
much smaller than one. In the limit that the amplitude of the fluctuations
remains small, we can successfully use linear perturbation theory.
To get a feel for how small density contrasts grow with time, consider
a particularly simple case. Start with a static, homogeneous, matter-only
universe with uniform mass density ρ̄. (At this point, we stumble over the
inconvenient fact that there’s no such thing as a static, homogeneous, matteronly universe. This is the awkward fact that inspired Einstein to introduce
the cosmological constant. However, there are conditions under which we
can consider some region of the universe to be approximately static and
homogeneous. For instance, the air in a closed room is approximately static
and homogeneous; it is stabilized by a pressure gradient with a scale length
which is much greater than the height of the ceiling.) In a region of the
universe which is approximately static and homogeneous, we add a small
amount of mass within a sphere of radius R, as seen in Figure 12.3, so that
the density within the sphere is ρ̄(1+δ), with δ ¿ 1. If the density excess δ is
uniform within the sphere, then the gravitational acceleration at the sphere’s
surface, due to the excess mass, will be
R̈ = −
G(∆M )
G
=− 2
2
R
R
µ
4π 3
R ρ̄δ
3
¶
,
(12.3)
260
CHAPTER 12. THE FORMATION OF STRUCTURE
or
R̈
4πGρ̄
=−
δ(t) .
R
3
(12.4)
Thus, a mass excess (δ > 0) will cause the sphere to collapse inward (R̈ < 0).
Equation (12.4) contains two unknowns, R(t) and δ(t). If we want to find
an explicit solution for δ(t), we need a second equation involving R(t) and
δ(t). Conservation of mass tells us that the mass of the sphere,
M=
4π
ρ̄[1 + δ(t)]R(t)3 ,
3
(12.5)
remains constant during the collapse. Thus, we can write another relation
between R(t) and δ(t) which must hold true during the collapse:
R(t) = R0 [1 + δ(t)]−1/3 ,
where
R0 ≡
Ã
3M
4π ρ̄
!1/3
= constant .
(12.6)
(12.7)
When δ ¿ 1, we may make the approximation
1
R(t) ≈ R0 [1 − δ(t)] .
3
(12.8)
Taking the second time derivative yields
1
1
R̈ ≈ − R0 δ̈ ≈ − Rδ̈ .
3
3
(12.9)
Thus, mass conservation tells us that
1
R̈
≈ − δ̈
R
3
(12.10)
in the limit that δ ¿ 1. Combining equations (12.4) and (12.10), we find a
tidy equation which tells us how the small overdensity δ evolves as the sphere
collapses:
δ̈ = 4πGρ̄δ .
(12.11)
The most general solution of equation (12.11) has the form
δ(t) = A1 et/tdyn + A2 e−t/tdyn ,
(12.12)
261
12.2. THE JEANS LENGTH
where the dynamical time for collapse is
tdyn
1
=
=
(4πGρ̄)1/2
Ã
c2
4πGε̄
!1/2
.
(12.13)
Note that the dynamical time depends only on ρ̄, and not on R. The constants A1 and A2 in equation (12.12) depend on the initial conditions of the
sphere. For instance, if the overdense sphere starts at rest, with δ̇ = 0 at
t = 0, then A1 = A2 = δ(0)/2. After a few dynamical times, however, only
the exponentially growing term of equation (12.12) is significant. Thus, gravity tends to make small density fluctuations in a static, pressureless medium
grow exponentially with time.
12.2
The Jeans length
The exponential growth of density perturbations is slightly alarming, at first
glance. For instance, the density of the air around you is ρ̄ ≈ 1 kg m−3 ,
yielding a dynamical time for collapse of tdyn ≈ 9 hours.2 What keeps small
density perturbations in the air from undergoing a runaway collapse over the
course of a few days? The answer, of course, is pressure. A non-relativistic
gas, as shown in section 4.3, has an equation-of-state parameter
w≈
kT
,
µc2
(12.14)
where T is the temperature of the gas and µ is the mean mass per gas
particle. Thus, the pressure of a ideal gas will never totally vanish, but will
only approach zero in the limit that the temperature approaches absolute
zero.
When a sphere of gas is compressed by its own gravity, a pressure gradient will build up which tends to counter the effects of gravity.3 However,
hydrostatic equilibrium, the state in which gravity is exactly balanced by a
pressure gradient, cannot always be attained. Consider an overdense sphere
2
Slightly longer if you are using this book for recreational reading as you climb Mount
Everest.
3
A star is the prime example of a dense sphere of gas in which the inward force of
gravity is balanced by the outward force provided by a pressure gradient.
262
CHAPTER 12. THE FORMATION OF STRUCTURE
with initial radius R. If pressure were not present, it would collapse on a
timescale
Ã
!1/2
c2
1
∼
.
(12.15)
tdyn ∼
(Gρ̄)1/2
Gε̄
If the pressure is nonzero, the attempted collapse will be countered by a
steepening of the pressure gradient within the perturbation. The steepening
of the pressure gradient, however, doesn’t occur instantaneously. Any change
in pressure travels at the sound speed.4 Thus, the time it takes for the
pressure gradient to build up in a region of radius R will be
tpre ∼
R
,
cs
(12.16)
where cs is the local sound speed. In a medium with equation-of-state parameter w > 0, the sound speed is
Ã
dP
cs = c
dε
!1/2
=
√
wc .
(12.17)
For hydrostatic equilibrium to be attained, the pressure gradient must build
up before the overdense region collapses, implying
tpre < tdyn .
(12.18)
Comparing equation (12.15) with equation (12.16), we find that for a density
perturbation to be stabilized by pressure against collapse, it must be smaller
than some reference size λJ , given by the relation
λJ ∼ cs tdyn ∼ cs
Ã
c2
Gε̄
!1/2
.
(12.19)
The length scale λJ is known as the Jeans length, after the astrophysicist
James Jeans, who was among the first to study gravitational instability in a
cosmological context. Overdense regions larger than the Jeans length collapse
under their own gravity. Overdense regions smaller than the Jeans length
merely oscillate in density; they constitute stable sound waves.
4
What is sound, after all, but a traveling change in pressure?
263
12.2. THE JEANS LENGTH
A more precise derivation of the Jeans length, including all the appropriate factors of π, yields the result
λJ = c s
Ã
πc2
Gε̄
!1/2
= 2πcs tdyn .
(12.20)
The Jeans length of the Earth’s atmosphere, for instance, where the sound
speed is a third of a kilometer per second and the dynamical time is nine
hours, is λJ ∼ 105 km, far longer than the scale height of the Earth’s atmosphere. You don’t have to worry about density fluctuations in the air
undergoing a catastrophic collapse.
To consider the behavior of density fluctuations on cosmological scales,
consider a spatially flat universe in which the mean density is ε̄, but which
contains density fluctuations with amplitude |δ| ¿ 1. The characteristic time
for expansion of such a universe is the Hubble time,
H
−1
=
Ã
3c2
8πGε̄
!1/2
.
(12.21)
Comparison of equation (12.13) with equation (12.21) reveals that the Hubble
time is comparable in magnitude to the dynamical time tdyn for the collapse
of an overdense region:
H −1 =
µ ¶1/2
3
2
tdyn ≈ 1.22tdyn .
(12.22)
The Jeans length in an expanding flat universe will then be
λJ = 2πcs tdyn = 2π
µ ¶1/2
3
2
cs
.
H
(12.23)
If we focus on one particular component of√the universe, with equation-ofstate parameter w and sound speed cs = wc, the Jeans length for that
component will be
µ ¶1/2
√ c
2
(12.24)
λJ = 2π
w .
3
H
Consider, for instance, the “radiation” component of the universe. With
w = 1/3, the sound speed in a gas of photons or other relativistic particles is
√
cs = c/ 3 ≈ 0.58c .
(12.25)
264
CHAPTER 12. THE FORMATION OF STRUCTURE
The Jeans length for radiation in an expanding universe is then
√
c
2π 2 c
≈ 3.0 .
(12.26)
λJ =
3 H
H
Density fluctuations in the radiative component will be pressure-supported if
they are smaller than three times the Hubble distance. Although a universe
containing nothing but radiation can have density perturbations smaller than
λJ ∼ 3c/H, they will be stable sound waves, and will not collapse under their
own gravity.
In order for a universe to have gravitationally collapsed structures much
smaller than
√ the Hubble distance, it must have a non-relativistic component, with w ¿ 1. The gravitational collapse of the baryonic component
of the universe is complicated by the fact that it was coupled to photons
until a redshift zdec ≈ zls ≈ 1100. In section 9.5, the Hubble distance
at the time of last scattering (effectively equal to the time of decoupling)
was shown to be c/H(zdec ) ≈ 0.2 Mpc. The energy density of baryons
at decoupling was εbary ≈ 2.8 × 1011 MeV m−3 , corresponding to a mass
density ρbary ≈ 5.0 × 10−19 kg m−3 , and the energy density of photons was
εγ ≈ 3.8 × 1011 MeV m−3 ≈ 1.4εbary .
Prior to decoupling, the photons, electrons, and baryons were all coupled
together to form a single photon-baryon fluid. Since the photons were still
dominant over the baryons at the time of decoupling, with εγ > εbary , we
can regard the baryons (with only mild exaggeration) as being a dynamically
insignificant contaminant in the photon-baryon fluid. Just before decoupling,
if we regard the baryons as a minor contaminant, the Jeans length of the
photon-baryon fluid was roughly the same as the Jeans length of a pure
photon gas:
λJ (before) ≈ 3c/H(zdec ) ≈ 0.6 Mpc ≈ 1.9 × 1022 m .
(12.27)
The baryonic Jeans mass, MJ , is defined as the mass of baryons contained
within a sphere of radius λJ ;
4π 3
MJ ≡ ρbary
λ
.
3 J
Immediately before decoupling, the baryonic Jeans mass was
µ
¶
4π
(1.9 × 1022 m)3
3
≈ 1.3 × 1049 kg ≈ 7 × 1018 M¯ .
MJ (before) ≈ 5.0 × 10−19 kg m−3
µ
¶
(12.28)
(12.29)
265
12.2. THE JEANS LENGTH
This is approximately 3×104 times greater than the estimated baryonic mass
of the Coma cluster, and represents a mass greater than the baryonic mass
of even the largest supercluster seen today.
Now consider what happens to the baryonic Jeans mass immediately after
decoupling. Once the photons are decoupled, the photons and baryons form
two separate gases, instead of a single photon-baryon fluid. The sound speed
in the photon gas is
√
(12.30)
cs (photon) = c/ 3 ≈ 0.58c .
The sound speed in the baryonic gas, by contrast, is
cs (baryon) =
Ã
kT
mc2
!1/2
c.
(12.31)
At the time of decoupling, the thermal energy per particle was kTdec ≈
0.26 eV, and the mean rest energy of the atoms in the baryonic gas was
mc2 = 1.22mp c2 ≈ 1140 MeV, taking into account the helium mass fraction
of Yp = 0.24. Thus, the sound speed of the baryonic gas immediately after
decoupling was
cs (baryon) ≈
µ
0.26 eV
1140 × 106 eV
¶1/2
c ≈ 1.5 × 10−5 c ,
(12.32)
only 5 kilometers per second. Thus, once the baryons were decoupled from
the photons, their associated Jeans length decreased by a factor
F =
1.5 × 10−5
cs (baryon)
≈
≈ 2.6 × 10−5 .
cs (photon)
0.58
(12.33)
Decoupling causes the baryonic Jeans mass to decrease by a factor F 3 ≈
1.8 × 10−14 , plummeting from MJ (before) ≈ 7 × 1018 M¯ to
MJ (after) = F 3 MJ (before) ≈ 1 × 105 M¯ .
(12.34)
This is comparable to the baryonic mass of the smallest dwarf galaxies known,
and is very much smaller than the baryonic mass of our own Galaxy, which
is ∼ 1011 M¯ .
The abrupt decrease of the baryonic Jeans mass at the time of decoupling
marks an important epoch in the history of structure formation. Perturbations in the baryon density, from supercluster scales down the the size of the
266
CHAPTER 12. THE FORMATION OF STRUCTURE
smallest dwarf galaxies, couldn’t grow in amplitude until the time of photon
decoupling, when the universe had reached the ripe old age of tdec ≈ 0.35 Myr.
After decoupling, the growth of density perturbations in the baryonic component was off and running. The baryonic Jeans mass, already small by
cosmological standards at the time of decoupling, dropped still further with
time as the universe expanded and the baryonic component cooled.
12.3
Instability in an expanding universe
Density perturbations smaller than the Hubble distance can grow in amplitude only when they are no longer pressure-supported. For the baryonic matter, this loss of pressure support happens abruptly at the time of decoupling,
when the Jeans length for baryons drops suddenly by a factor F ∼ 3 × 10−5 .
For the dark matter, the loss of pressure support occurs more gradually, as
the thermal energy of the dark matter particles drops below their rest energy.
When considering the Cosmic Neutrino Background, for instance, which has
a temperature comparable to the Cosmic Microwave Background, we found
(see equation 5.18) that neutrinos of mass mν became non-relativistic at a
redshift
1
mν c2
1+z = ≈
.
(12.35)
a
5 × 10−4 eV
Thus, if the universe contains a Cosmic WIMP Background comparable in
temperature to the Cosmic Neutrino Background, the WIMPs, if they have
a mass mW c2 À 2 eV, would have become non-relativistic long before the
time of radiation-matter equality at zrm ≈ 3570.
Once the pressure (and hence the Jeans length) of some component becomes negligibly small, does this imply that the amplitude of density fluctuations is free to grow exponentially with time? Not necessarily. The
analysis of section 12.1, which yielded δ ∝ exp(t/tdyn ), assumed that the
universe was static as well as pressureless. In an expanding Friedmann
universe, the timescale for the growth of a density perturbation by selfgravity, tdyn ∼ (c2 /Gε̄)1/2 , is comparable to the timescale for expansion,
H −1 ∼ (c2 /Gε̄)1/2 . Self-gravity, in the absence of global expansion, causes
overdense regions to become more dense with time. The global expansion of
the universe, in the absence of self-gravity, causes overdense regions to become less dense with time. Because the timescales for these two competing
processes are similar, they must both be taken into account when computing
12.3. INSTABILITY IN AN EXPANDING UNIVERSE
267
the time evolution of a density perturbation.
To get a feel how small density perturbations in an expanding universe
evolve with time, let’s do a Newtonian analysis of the problem, similar in
spirit to the Newtonian derivation of the Friedmann equation given in Chapter 4. Suppose you are in a universe filled with pressureless matter which
has mass density ρ̄(t). As the universe expands, the density decreases at the
rate ρ̄(t) ∝ a(t)−3 . Within a spherical region of radius R, a small amount of
matter is added, or removed, so that the density within the sphere is
ρ(t) = ρ̄(t)[1 + δ(t)] ,
(12.36)
with |δ| ¿ 1. (In performing a Newtonian analysis of this problem, we
are implicitly assuming that the radius R is small compared to the Hubble
distance and large compared to the Jeans length.) The total gravitational
acceleration at the surface of the sphere will be
GM
G
R̈ = − 2 = − 2
R
R
µ
4π
4π
4π 3
ρR = − Gρ̄R −
G(ρ̄δ)R .
3
3
3
¶
(12.37)
The equation of motion for a point at the surface of the sphere can then be
written in the form
4π
4π
R̈
= − Gρ̄ −
Gρ̄δ .
(12.38)
R
3
3
Mass conservation tells us that the mass inside the sphere,
M=
4π
ρ̄(t)[1 + δ(t)]R(t)3 ,
3
(12.39)
remains constant as the sphere expands. Thus,
or, since ρ̄ ∝ a−3 ,
R(t) ∝ ρ̄(t)−1/3 [1 + δ(t)]−1/3 ,
(12.40)
R(t) ∝ a(t)[1 + δ(t)]−1/3 .
(12.41)
R̈
ä 1
2 ȧ
= − δ̈ −
δ̇ ,
R
a 3
3a
(12.42)
That is, if the sphere is slightly overdense, its radius will grow slightly less
rapidly than the scale factor a(t). If the sphere is slightly underdense, it will
grow slightly more rapidly than the scale factor.
Taking two time derivatives of equation (12.41) yields
268
CHAPTER 12. THE FORMATION OF STRUCTURE
when |δ| ¿ 1. Combining equations (12.38) and (12.42), we find
ä 1
2 ȧ
4π
4π
− δ̈ −
δ̇ = − Gρ̄ −
Gρ̄δ .
a 3
3a
3
3
(12.43)
When δ = 0, equation (12.43) reduces to
ä
4π
= − Gρ̄ ,
a
3
(12.44)
which is the correct acceleration equation for a homogeneous, isotropic universe containing nothing but pressureless matter (compare to equation 4.44).
By subtracting equation (12.44) from equation (12.43) to leave only the terms
linear in the perturbation δ, we find the equation which governs the growth
of small perturbations:
2 ȧ
4π
1
δ̇ = − Gρ̄δ ,
− δ̈ −
3
3a
3
(12.45)
δ̈ + 2H δ̇ = 4πGρ̄δ ,
(12.46)
or
remembering that H ≡ ȧ/a. In a static universe, with H = 0, equation (12.46) reduces to the result which we have already found in equation (12.11):
δ̈ = 4πGρ̄δ .
(12.47)
The additional term, ∝ H δ̇, found in an expanding universe, is sometimes
called the “Hubble friction” term; it acts to slow the growth of density perturbations in an expanding universe.
A fully relativistic calculation for the growth of density perturbations
yields the more general result
δ̈ + 2H δ̇ =
4πG
ε̄m δ .
c2
(12.48)
This form of the equation can be applied to a universe which contains components with non-negligible pressure, such as radiation (w = 1/3) or a cosmological constant (w = −1). In multiple-component universes, however,
it should be remembered that δ represents the fluctuation in the density of
matter alone. That is,
εm − ε̄m
δ=
,
(12.49)
ε̄m
12.3. INSTABILITY IN AN EXPANDING UNIVERSE
269
where ε̄m (t), the average matter density, might be only a small fraction of
ε̄(t), the average total density. Rewritten in terms of the density parameter
for matter,
ε̄m
8πGε̄m
Ωm =
= 2 2 ,
(12.50)
εc
3c H
equation (12.48) takes the form
3
δ̈ + 2H δ̇ − Ωm H 2 δ = 0 .
2
(12.51)
During epochs when the universe is not dominated by matter, density perturbations in the matter do not grow rapidly in amplitude. Take, for instance,
the early radiation-dominated phase of the universe. During this epoch,
Ωm ¿ 1 and H = 1/(2t), meaning that equation (12.51) takes the form
1
δ̈ + δ̇ ≈ 0 ,
t
(12.52)
which has a solution of the form
δ(t) ≈ B1 + B2 ln t .
(12.53)
During the radiation-dominated epoch, density fluctuations in the dark matter grew only at a logarithmic rate. In the far future, if the universe is indeed
dominated by a cosmological constant, the density parameter for matter will
again be negligibly small, the Hubble parameter will have the constant value
H = HΛ , and equation (12.51) will take the form
δ̈ + 2HΛ δ̇ ≈ 0 ,
(12.54)
which has a solution of the form
δ(t) ≈ C1 + C2 e−2HΛ t .
(12.55)
In a lambda-dominated phase, therefore, fluctuations in the matter density reach a constant fractional amplitude, while the average matter density
plummets at the rate ε̄m ∝ e−3HΛ t .
It is only when matter dominates the energy density that fluctuations in
the matter density can grow at a significant rate. In a flat, matter-dominated
universe, Ωm = 1, H = 2/(3t), and equation (12.51) takes the form
δ̈ +
4
2
δ̇ − 2 δ = 0 .
3t
3t
(12.56)
270
CHAPTER 12. THE FORMATION OF STRUCTURE
If we guess that the solution to the above equation has the power-law form
Dtn , plugging this guess into the equation yields
n(n − 1)Dtn−2 +
2
4
nDtn−1 − 2 Dtn = 0 ,
3t
3t
(12.57)
or
4
2
n(n − 1) + n − = 0 .
(12.58)
3
3
The two possible solutions for this quadratic equation are n = −1 and
n = 2/3. Thus, the general solution for the time evolution of density perturbations in a spatially flat, matter-only universe is
δ(t) ≈ D1 t2/3 + D2 t−1 .
(12.59)
The values of D1 and D2 are determined by the initial conditions for δ(t).
The decaying mode, ∝ t−1 , eventually becomes negligibly small compared to
the growing mode, ∝ t2/3 . When the growing mode is the only survivor, the
density perturbations in a flat, matter-only universe grow at the rate
δ ∝ t2/3 ∝ a(t) ∝
1
1+z
(12.60)
as long as |δ| ¿ 1.
When an overdense region attains an overdensity δ ∼ 1, its evolution can
no longer be treated with a simple linear perturbation approach. Studies
of the nonlinear evolution of structure are commonly made using numerical
computer simulations, in which the matter filling the universe is modeled as
a distribution of point masses interacting via Newtonian gravity. In these
simulations, as in the real universe, when a region reaches an overdensity
δ ∼ 1, it breaks away from the Hubble flow and collapses. After one or
two oscillations in radius, the overdense region attains virial equilibrium as a
gravitationally bound structure. If the baryonic matter within the structure
is able to cool efficiently (by bremsstrahlung or some other process) it will radiate away energy and fall to the center. The centrally concentrated baryons
can then proceed to form stars, becoming the visible portions of galaxies that
we see today. The less concentrated nonbaryonic matter forms the dark halo
within which the stellar component of the galaxy is embedded.
If baryonic matter were the only type of non-relativistic matter in the universe, then density perturbations could have started to grow at zdec ≈ 1100,
12.3. INSTABILITY IN AN EXPANDING UNIVERSE
271
Figure 12.4: A highly schematic drawing of how density fluctuations in different components of the universe evolve with time.
and they could have grown in amplitude only by a factor ∼ 1100 by the
present day. However, the mominant form of non-relativistic matter is dark
matter. The density perturbations in the dark matter started to grow effectively at zrd ≈ 3570. At the time of decoupling, the baryons fell into the
preexisting gravitational wells of the dark matter. The situation is schematically illustrated in Figure 12.4. Having nonbaryonic dark matter allows the
universe to get a “head start” on structure formation; perturbations in the
matter density can start growing at zrd ≈ 3570 rather than zdec ≈ 1100, as
they would in a universe without dark matter.
272
12.4
CHAPTER 12. THE FORMATION OF STRUCTURE
The power spectrum
When deriving equation (12.46), which determines the growth rate of density
perturbations in a Newtonian universe, I assumed that the perturbation was
spherically symmetric. In fact, equation (12.46) and its relativistically correct
brother, equation (12.48), both apply to low-amplitude perturbations of any
shape. This is fortunate, since the density perturbations in the real universe
are not spherically symmetric. The bubbly structure shown in redshift maps
of galaxies, such as Figure 12.1, has grown from the density perturbations
which were present when the universe became matter dominated. Great oaks
from tiny acorns grow – but then, great pine trees from tiny pinenuts grow.
By looking at the current large scale structure (the “tree”), we can deduce
the properties of the early, low-amplitude, density fluctuations (the “nut”). 5
Let us consider the properties of the early density fluctuations at some
time ti when they were still very low in amplitude (|δ| ¿ 1). As long as the
density fluctuations are small in amplitude, the expansion of the universe is
still nearly isotropic, and the geometry of the universe is still well described
by the Robertson-Walker metric (equation 3.25):
ds2 = −c2 dt2 + a(t)2 [dr2 + Sκ (r)2 dΩ2 ] .
(12.61)
Under these circumstances, it is useful to set up a comoving coordinate
system. Choose some point as the origin. In a universe described by the
Robertson-Walker metric, as shown in section 3.4, the proper distance of any
point from the origin can be written in the form
dp (ti ) = a(ti )r ,
(12.62)
where the comoving distance r is what the proper distance would be at the
present day (a = 1) if the expansion continued to be perfectly isotropic.
If we label each bit of matter in the universe with its comoving coordinate
position ~r = (r, θ, φ), then ~r will remain very nearly constant as long as
|δ| ¿ 1. Thus, when considering the regime where density fluctuations
are small, cosmologists typically consider δ(~r), the density fluctuation at a
comoving location ~r, at some time ti . (The exact value of ti doesn’t matter,
as long as it’s a time after the density perturbations are in place, but before
5
At the risk of carrying the arboreal analogy too far, I should mention that the temperature fluctuations of the Cosmic Microwave Background, as shown in Figures 9.2 and
9.5, offer us a look at the “sapling”.
273
12.4. THE POWER SPECTRUM
they reach an amplitude |δ| ∼ 1. Switching to a different value of ti , under
these restrictions, simply changes the amplitude of δ(~r), and not its shape.)
When discussing the temperature fluctuations of the Cosmic Microwave
Background, back in Chapter 9, I pointed out that cosmologists weren’t interested in the exact pattern of hot and cold spots on the last scattering
surface, but rather in the statistical properties of the field δT /T (θ, φ). Similarly, cosmologists are not interested in the exact locations of the density
maxima and minima in the early universe, but rather in the statistical properties of the field δ(~r). When studying the temperature fluctuations of the
CMB, it is useful to expand δT /T (φ, θ) in spherical harmonics. A similar
decomposition of δ(~r) is also useful. Since δ is defined in three-dimensional
space (rather than on the surface of a sphere), a useful expansion of δ is in
terms of Fourier components.
Within a large comoving box, of comoving volume V , the density fluctuation field δ(~r) can be expressed as
δ(~r) =
V Z
~
δ~k e−ik·~r d3 k ,
3
(2π)
(12.63)
where the individual Fourier components δ~k are found by performing the
integral
1 Z
~
δ~k =
δ(~r)eik·~r d3 r .
(12.64)
V
In performing the Fourier transform, you are breaking up the function δ(~r)
into an infinite number of sine waves, each with comoving wavenumber ~k and
comoving wavelength λ = 2π/k. If you have complete, uncensored knowledge
of δ(~r), you can compute all the Fourier components δ~k ; conversely, if you
know all the Fourier components, you can reconstruct the density field δ(~r).
Each Fourier component is a complex number, which can be written in
the form
δ~k = |δ~k |eiφ~k .
(12.65)
When |δ~k | ¿ 1, then each Fourier component obeys equation (12.51),
3
δ̈~k + 2H δ̇~k − Ωm H 2 δ~k = 0 ,
2
(12.66)
as long as the proper wavelength, a(t)2π/k, is large compared to the Jeans
274
CHAPTER 12. THE FORMATION OF STRUCTURE
length and small compared to the Hubble distance c/H.6 The phase φ~k
remains constant as long as the amplitude |δ~k | remains small. Even after
fluctuations with a short proper wavelength have reached |δ~k | ∼ 1 and collapsed, the growth of the longer wavelength perturbations is still described
by equation (12.66). This means, helpfully enough, that we can use linear
perturbation theory to study the growth of very large scale structure even after smaller structures, such as galaxies and clusters of galaxies, have already
collapsed.
The mean square amplitude of the Fourier components defines the power
spectrum:
P (k) = h|δ~k |2 i ,
(12.67)
where the average is taken over all possible orientations of the wavenumber
~k. (If δ(~r) is isotropic, then no information is lost, statistically speaking,
if we average the power spectrum over all angles.) When the phases φ~k of
the different Fourier components are uncorrelated with each other, then δ(~r)
is called a Gaussian field. If a Gaussian field is homogeneous and isotropic,
then all its statistical properties are summed up in the power spectrum P (k).
If δ(~r) is a Gaussian field, then the value of δ at a randomly selected point
is drawn from the Gaussian probability distribution
δ2
1
exp − 2
p(δ) = √
2σ
2πσ
Ã
!
,
(12.68)
where the standard deviation σ can be computed from the power spectrum:
V Z∞
V Z
3
P (k)d k = 2
P (k)k 2 dk .
σ=
(2π)3
2π 0
(12.69)
The study of Gaussian density fields is of particular interest to cosmologists
because most inflationary scenarios predict that the density fluctuations created by inflation (see section 11.5) will be an isotropic, homogeneous Gaussian field. In addition, the expected power spectrum for the inflationary
fluctuations has a scale-invariant, power-law form:
P (k) ∝ k n ,
6
(12.70)
When a sine wave perturbation has a wavelength large compared to the Hubble distance, its crests are not causally connected to its troughs. As long as the crests remain
out of touch with the troughs (that is, as long as a(t)2π/k > c/H(t)), the amplitude of a
perturbation grows at the rate δ(t) ∝ a(t).
275
12.4. THE POWER SPECTRUM
with the favored value of the power-law index being n = 1. The preferred
power spectrum, P (k) ∝ k, is often referred to as a Harrison-Zel’dovich
spectrum.
What would a universe with P (k) ∝ k n look like? Imagine going through
such a universe and marking out randomly located spheres of comoving radius L. The mean mass of each sphere (considering only the non-relativistic
matter which it contains) will be
hM i =
4π 3 εm,0
L 2 .
3
c
(12.71)
However, the actual mass of each sphere will vary; some spheres will be
slightly underdense, and others will be slightly overdense. The mean square
density fluctuation of the mass inside each sphere is a function of the power
spectrum and of the size of the sphere:
*Ã
M − hM i
hM i
!2 +
∝ k 3 P (k) ,
(12.72)
where the comoving wavenumber associated with the sphere is k = 2π/L.
Thus, if the power spectrum has the form P (k) ∝ k n , the root mean square
mass fluctuation within spheres of comoving radius L will be
δM
≡
M
*Ã
M − hM i
hM i
!2 +1/2
∝ L−(3+n)/2 .
(12.73)
This can also be expressed in the form δM/M ∝ M −(3+n)/6 . For n < −3, the
mass fluctuations diverge on large scales, which would be Bad News for our
assumption of homogeneity on large scales. (Note that if you scattered point
masses randomly throughout the universe, so that they formed a Poisson distribution, you would expect mass fluctuations of amplitude δM/M ∝ N −1/2 ,
where N is the expected number of point masses within the sphere. Since
the average mass within a sphere is M ∝ N , a Poisson point distribution has δM/M ∝ M −1/2 , or n = 0. The Harrison-Zel’dovich spectrum,
with n = 1, thus will produce more power on small length scales than a
Poisson distribution of points.) Note that the potential fluctuations associated with mass fluctuations on a length scale L will have an amplitude
δΦ ∼ GδM/L ∝ δM/M 1/3 ∝ M (1−n)/6 . Thus, the Harrison-Zel’dovich spectrum, with n = 1, is the only power law which prevents the divergence of the
potential fluctuations on both large and small scales.
276
12.5
CHAPTER 12. THE FORMATION OF STRUCTURE
Hot versus cold
Immediately after inflation, the expected power spectrum for density perturbations has the form P (k) ∝ k n , with an index n = 1 being predicted by
most inflationary models. However, the shape of the power spectrum will be
modified between the end of inflation at tf and the time of radiation-matter
equality at trm ≈ 4.7 × 104 yr. The shape of the power spectrum at trm , when
density perturbations start to grow significantly in amplitude, depends on
the properties of the dark matter. More specifically, it depends on whether
the dark matter is predominantly cold dark matter or hot dark matter.
Cold dark matter consists of particles which were non-relativistic at the
time they decoupled from the other components of the universe. For instance,
WIMPs would have had thermal velocities much smaller than c at the time
they decoupled, and hence qualify as cold dark matter. If any primordial
black holes had formed in the early universe, their peculiar velocities would
have been much smaller than c at the time they formed; thus primordial black
holes would also act as cold dark matter. Axions are a type of elementary
particle first proposed by particle physicists for non-cosmological purposes.
If they exist, however, they would have formed out of equilibrium in the early
universe, with very low thermal velocities. Thus, axions would act as cold
dark matter, as well.
Hot dark matter, by contrast, consists of particles which were relativistic
at the time they decoupled from the other components of the universe, and
which remained relativistic until the mass contained within a Hubble volume
(a sphere of proper radius c/H) was large compared to the mass of a galaxy.
In the Benchmark Model, the Hubble distance at the time of radiation-matter
equality was
3/2
c Ωr,0
c
=√
≈ 1.8ctrm ≈ 0.026 Mpc ,
H(trm )
2H0 Ω2m,0
so the mass within a Hubble volume at that time was
√
3/2
4π c3 Ωm,0 ρc,0
2π c3 Ωr,0
=
ρc,0 ≈ 1.4 × 1017 M¯ ,
3 2
3
3
3 H(trm )
arm
3 H0 Ωm,0
(12.74)
(12.75)
much larger than the mass of even a fairly large galaxy such as our own
(Mgal ≈ 1012 M¯ ). Thus, a weakly interacting particle which remains relativistic until the universe becomes matter-dominated will act as hot dark
277
12.5. HOT VERSUS COLD
matter. For instance, neutrinos decoupled at t ∼ 1 s, when the universe had
a temperature kT ∼ 1 MeV. Thus, a neutrino with mass mν c2 ¿ 1 MeV
was hot enough to be relativistic at the time it decoupled. Moreover, as
discussed in section 5.1, a neutrino with mass mν c2 < 2 eV doesn’t become
non-relativistic until after radiation-matter equality, and hence qualifies as
hot dark matter.7
To see how the existence of hot dark matter modifies the spectrum of
density perturbations, consider what would happen in a universe filled with
weakly interacting particles which are relativistic at the time they decouple.
The initially relativistic particles cool as the universe expands, until their
thermal velocities drop well below c when 3kT ∼ mh c2 . This happens at a
temperature
!
Ã
mh c2
mh c2
.
(12.76)
∼ 8000 K
Th ∼
3k
2 eV
In the radiation-dominated universe, this corresponds to a cosmic time (equation 10.2)
Ã
!−2
mh c2
12
th ∼ 2 × 10 s
.
(12.77)
2 eV
Prior to the time th , the hot dark matter particles move freely in random
directions with a speed close to that of light. This motion, called free streaming, acts to wipe out any density fluctuations present in the hot dark matter.
Thus, the net effect of free streaming in the hot dark matter is to wipe out
any density fluctuations whose wavelength is smaller than ∼ cth . When the
hot dark matter particles become non-relativistic, there will be no density
fluctuations on scales smaller than the physical scale
λmin
mh c2
∼ cth ∼ 20 kpc
2 eV
Ã
!−2
,
(12.78)
corresponding to a comoving length scale
Lmin
7
mh c2
Th
λmin
∼
λmin ∼ 60 Mpc
=
a(th )
2.725 K
2 eV
Ã
!−1
.
(12.79)
It may seem odd to refer to neutrinos as “hot” dark matter, when the temperature
of the Cosmic Neutrino Background is only two degrees above absolute zero. The label
“hot”, in this case, simply means that the neutrinos were hot enough to be relativistic
back in the radiation-dominated era.
278
CHAPTER 12. THE FORMATION OF STRUCTURE
The total amount of matter within a sphere of comoving radius Lmin is
Mmin
4π 3
mh c2
16
L Ωm,0 ρc,0 ∼ 5 × 10 M¯
=
3 min
2 eV
Ã
!−3
,
(12.80)
assuming Ωm,0 = 0.3. If the dark matter is contributed by neutrinos with
rest energy of a few electron volts, then the free streaming will wipe out all
density fluctuations smaller than superclusters.
The upper panel of Figure 12.5 shows the power spectrum of density fluctuations in hot dark matter, once the hot dark matter has cooled enough to
become non-relativistic. Note that for wavenumbers k ¿ 2π/Lmin , the power
spectrum of hot dark matter (shown as the dotted line) is indistinguishable
from the original P ∝ k spectrum (shown as the dashed line). However, the
free streaming of the hot dark matter results in a severe loss of power for
wavenumbers k À 2π/Lmin . The lower panel of Figure 12.5 shows that the
root mean square mass fluctuations in hot dark matter, δM/M ∝ (k 3 P )1/2 ,
have a maximum amplitude at a mass scale M ∼ 1016 M¯ . This implies that
in a universe filled with hot dark matter, the first structures to collapse are
the size of superclusters. Smaller structures, such as clusters and galaxies
then form by fragmentation of the superclusters. (This scenario, in which
the largest observable structures form first, is called the top-down scenario.)
If most of the dark matter in the universe were hot dark matter, such
as neutrinos, then we would expect the oldest structures in the universe to
be superclusters, and that galaxies would be relatively young. In fact, the
opposite seems to be true in our universe. Superclusters are just collapsing
today, while galaxies have been around since at least z ∼ 6, when the universe
was less than a gigayear old. Thus, most of the dark matter in the universe
must be cold dark matter, for which free streaming has been negligible.
The evolution of the power spectrum of cold dark matter, given the absence of free streaming, is quite different from the evolution of the power
spectrum for hot dark matter. Remember, when the universe is radiationdominated, density fluctuations δ~k in the dark matter do not grow appreciably in amplitude, as long as their proper wavelength a(t)2π/k is small
compared to the Hubble distance c/H(t). However, when the proper wavelength of a density perturbation is large compared to the Hubble distance,
its amplitude will be able to increase, regardless of whether the universe is
radiation-dominated or matter-dominated. If the cold dark matter consists
of WIMPs, they decouple from the radiation at a time td ∼ 1 s, when the
279
12.5. HOT VERSUS COLD
1
.1
P∝k
P
.01
.001
CDM
.0001
10
−5
10
−6
10
−7
.001
HDM
.01
.1
−1
k [Mpc ]
1
10
10
1
3
(k P)
1/2
.1
.01
.001
.0001
10
−5
12
14
16
log10M
18
20
Figure 12.5: Upper panel – The power spectrum at the time of radiationmatter equality for cold dark matter (solid line) and for hot dark matter
(dotted line). The initial power spectrum produced by inflation (dashed
line) is assumed to have the form P (k) ∝ k. The normalization of the power
spectrum is arbitrary. Lower panel – The root mean square mass fluctuations,
δM/M ∝ (k 3 P )1/2 , are shown as a function of M ∝ k −3 (masses are in units
of M¯ ). The line types are the same as in the upper panel.
280
CHAPTER 12. THE FORMATION OF STRUCTURE
scale factor is ad ∼ 3 × 10−10 . At the time of WIMP decoupling, the Hubble
distance is c/H ∼ 2ctd ∼ 6×108 m, corresponding to a comoving wavenumber
kd ∼
2πad
∼ 105 Mpc−1 .
2ctd
(12.81)
Thus, density fluctuations with a wavenumber k < kd will have a wavelength
greater than the Hubble distance at the time of WIMP decoupling, and will
be able to grow freely in amplitude, as long as their wavelength remains
longer than the Hubble distance. Density fluctuations with k > kd will
remain frozen in amplitude until matter starts to dominate the universe at
trm ≈ 4.7 × 104 yr, when the scale factor has grown to arm ≈ 2.8 × 10−4 .
At the time of radiation-matter equality, the Hubble distance, as given in
equation (12.74), is c/H ≈ 1.8ctrm ≈ 0.026 Mpc, corresponding to a comoving
wavenumber
2πarm
krm ≈
≈ 0.07 Mpc−1 .
(12.82)
1.8ctrm
Thus, density fluctuations with a wavenumber k < krm ≈ 0.07 Mpc−1 will
grow steadily in amplitude during the entire radiation-dominated era, and
for wavenumbers k < krm ≈ 0.07 Mpc−1 , the power spectrum for cold dark
matter retains the original P (k) ∝ k form which it had immediately after
inflation (see the upper panel of Figure 12.5).
By contrast, cold dark matter density perturbations with a wavenumber
kd > k > krm will be able to grow in amplitude only until their physical wavelength a(t)/(2πk) ∝ t1/2 is smaller than the Hubble distance c/H(t) ∝ t. At
that time, their amplitude will be frozen until the time trm , when matter
dominates, and density perturbations smaller than the Hubble distance are
free to grow again. Thus, for wavenumbers k > krm , the power spectrum
for cold dark matter is suppressed in amplitude, with the suppression being
greatest for the largest wavenumbers (corresponding to shorter wavelengths,
which come within the horizon at an earlier time). The top panel of Figure 12.5 shows, as the solid line, the power spectrum for cold dark matter at
the time of radiation-matter equality. Note the broad maximum in the power
spectrum at k ∼ krm ≈ 0.07 Mpc−1 . The root mean square mass fluctuations
in the cold dark matter, shown in the bottom panel of Figure 12.5 are largest
in amplitude for the smallest mass scales. This implies that in a universe
filled with cold dark matter, the first objects to form are the smallest, with
galaxies forming first, then clusters, then superclusters. This scenario, called
12.5. HOT VERSUS COLD
281
the bottom-up scenario, is consistent with the observed ages of galaxies and
superclusters.
Assuming that the dark matter consists of nothing but hot dark matter
gives a poor fit to the observed large scale structure of the universe. Assuming
that the dark matter is purely cold dark matter gives a much better fit.
However, there is strong evidence that neutrinos do have some mass, and
thus that the universe contains at least some hot dark matter. Cosmologists
studying the large scale structure of the universe can adjust the assumed
power spectrum of the dark matter, by mixing together hot and cold matter.
(It’s a bit like adjusting the temperature of your bath by tweaking the hot
and cold water knobs.) Comparison of the assumed power spectrum to the
observed large scale structure (as seen, for instance, in figure 12.1) reveals
that ∼ 13% or less of the matter in the universe consists of hot dark matter.
For Ωm,0 = 0.3, this implies ΩHDM,0 ≤ 0.04. If there were more hot dark
matter than this amount, free streaming of the hot dark matter particles
would make the universe too smooth on small scales. Some like it hot, but
most like it cold – the majority of the dark matter in the universe must be
cold dark matter.
Suggested reading
[Full references are given in the “Annotated Bibliography” on page 286.]
Liddle & Lyth (2000): The origin of density perturbations during the inflationary era, and their growth thereafter.
Longair (1998): For those who want to know more about galaxy formation,
and how it ties into cosmology.
Rich (2001), ch. 7: The origin and evolution of density fluctuations.
Problems
(12.1) Consider a spatially flat, matter-dominated universe (Ω = Ωm = 1)
which is contracting with time. What is the functional form of δ(t) in
such a universe?
282
CHAPTER 12. THE FORMATION OF STRUCTURE
(12.2) Consider an empty, negatively curved, expanding universe, as described in section 5.2. If a dynamically insignificant amount of matter
(Ωm ¿ 1) is present in such a universe, how do density fluctuations in
the matter evolve with time? That is, what is the functional form of
δ(t)?
(12.3) A volume containing a photon-baryon fluid is adiabatically expanded
or compressed. The energy density of the fluid is ε = εγ + εbary , and
the pressure is P = Pγ = εγ /3. What is dP/dε for the photon-baryon
fluid? What is the sound speed, cs ? In equation (12.27), how large of
an error did I make in my estimate of λJ (before) by ignoring the effect
of the baryons on the sound speed of the photon-baryon fluid?
(12.4) Suppose that the stars in a disk galaxy have a constant orbital speed
v out to the edge of its spherical dark halo, at a distance Rhalo from
the galaxy’s center. What is the average density ρ̄ of the matter in the
galaxy, including its dark halo? (Hint: go back to section 8.2.) What
is the value of ρ̄ for our Galaxy, assuming v = 220 km s−1 and Rhalo =
100 kpc? If a bound structure, such as a galaxy, forms by gravitational
collapse of an initially small density
perturbation, the minimum time
√
for collapse is tmin ≈ tdyn ≈ 1/ Gρ̄. Show that tmin ≈ Rhalo /v for a
disk galaxy. What is tmin for our own Galaxy? What is the maximum
possible redshift at which you would expect to see galaxies comparable
in v and Rhalo to our own Galaxy? (Assume the Benchmark Model is
correct.)
(12.5) Within the Coma cluster, as discussed in section 8.3, galaxies have
a root mean square velocity of hv 2 i1/2 ≈ 1520 km s−1 relative to the
center of mass of the cluster; the half-mass radius of the Coma cluster
is rh ≈ 1.5 Mpc. Using arguments similar to those of the previous
problem, compute the minimum time tmin required for the Coma cluster
to form by gravitational collapse.
(12.6) Derive equation (12.74), giving the Hubble distance at the time of
radiation-matter equality. What was the Hubble distance at the time of
matter-lambda equality, in the Benchmark Model? How much matter
was contained within a Hubble volume at the time of matter-lambda
equality?
12.5. HOT VERSUS COLD
283
(12.7) Warm dark matter is defined as matter which became non-relativistic
when the amount of matter within a Hubble volume had a mass comparable to that of a galaxy. In the Benchmark Model, at what time
tWDM was the mass contained within a Hubble volume equal to Mgal =
1012 M¯ ? If the warm dark matter particles have a temperature equal
to that of the cosmic neutrino background, what mass must they have
in order to have become non-relativistic at t ∼ tWDM ?
Epilogue
A book dealing with an active field like cosmology can’t really have a neat,
tidy ending. Our understanding of the universe is still growing and evolving.
During the twentieth century, the growing weight of evidence pointed toward
the Hot Big Bang model, in which the universe started in a hot, dense state,
but gradually cooled as it expanded. At the end of the twentieth century
and the beginning of the twenty-first, cosmological evidence was gathered at
an increasing rate, refining our knowledge of the universe. As I write this
epilogue, on a sunny spring day in the year 2002, the available evidence is
explained by a Benchmark Model which is spatially flat and which has an
expansion which is currently accelerating. It seems that 70% of the energy
density of the universe is contributed by a cosmological constant (or other
form of “dark energy” with negative pressure). Only 30% of the energy
density is contributed by matter (and only 4% is contributed by the familiar
baryonic matter of which you and I are made).
However, many questions about the cosmos remain unanswered. Here are
a few of the questions that currently nag at cosmologists:
• What are the precise values of cosmological parameters such as H 0 , q0 ,
Ωm,0 , and ΩΛ,0 ? Much effort has been invested in determining these
parameters, but they are still not pinned down precisely.
• What is the dark matter? It can’t be made entirely of baryons. It
can’t be made entirely of neutrinos. Most of the dark matter must be
in the form of some exotic stuff which has not yet been detected in
laboratories.
• What is the “dark energy”? Is it vacuum energy which plays the role of
a cosmological constant, or is it some other component of the universe
with −1 < w < −1/3? If it is vacuum energy, is it provided by a false
284
285
vacuum, driving a temporary inflationary stage, or are we finally seeing
the true vacuum energy?
• What drove inflation during the early universe? Our knowledge of the
particle physics behind inflation is still sadly incomplete. Indeed, some
cosmologists pose the questions, “Did inflation take place at all during
the early universe? Is there another way to resolve the flatness, horizon,
and monopole problems?”
• Why is the universe expanding? At one level, this question is easily
answered. The universe is expanding today because it was expanding
yesterday. It was expanding yesterday because it was expanding the day
before yesterday... However, when you extrapolate back to the Planck
time, you find that the universe was expanding then with a Hubble
parameter H ∼ 1/tP . What determined this set of initial conditions?
In other words, “What put the Bang in the Big Bang?”
The most interesting questions, however, are those which we are still too
ignorant to pose correctly. For instance, in ancient Egypt, a list of unanswered questions in cosmology might have included “How high is the dome
which makes up the sky?” and “What’s the dome made of?” Severely erroneous models of the universe obviously give rise to irrelevant questions.
The exciting, unsettling possibility exists that future observations will render the now-promising Benchmark Model obsolete. I hope, patient reader,
that learning about cosmology from this book has encouraged you to become
a cosmologist yourself, and to join the scientists who are laboring to make
my book a quaint, out-of-date relic from a time when the universe was poorly
understood.
Annotated Bibliography
Works described as ‘Popular’ contain little or no math. Those described as
‘Intermediate’ are at roughly the same level as this book. Those described as
‘Advanced’ have a higher level of mathematical and physical sophistication,
appropriate for study at a graduate level.
Popular
• Harrison, E. R. 1987, Darkness at Night: A Riddle of the Universe
(Cambridge: Harvard University Press) A comprehensive discussion of
Olbers’ Paradox and its place in the history of cosmology.
• Kragh, H. 1996, Cosmology and Controversy (Princeton: Princeton
University Press) A well-reseached history of the Big Bang vs. Steady
State debate. A fascinating book if you are at all interested in the
sociology of science.
• Silk, J. 2001, The Big Bang (third edition) (New York: W. H. Freeman
& Co.) A broad overview of cosmology. Although aimed at a popular
audience (with all mathematical formulas banished to an appendix), it
doesn’t skimp on the physics.
• Weinberg, S. 1993, The First Three Minutes (revised edition) (Perseus
Books) A classic of popular science. Weinberg’s revision has brought
the original 1977 version more nearly up-to-date.
Intermediate
• Bernstein, J. 1995, Introduction to Cosmology (Englewood Cliffs, NJ:
Prentice Hall) Has a slightly greater emphasis on particle physics than
286
287
most cosmology texts.
• Coles, P. 1999, The Routledge Critical Dictionary of the New Cosmology
(New York: Routledge) In addition to a dictionary of cosmology-related
terms, from “absorption line” to “Zel’dovich-Sunyaev effect”, this book
also contains longer essays on cosmological topics of current interest.
• Cox, A. N., ed. 2000, Allen’s Astrophysical Quantities (fourth edition)
(New York: Springer-Verlag) A standard reference book of astronomically relevant data, from the Euler vectors of the Nazca plate to the
intensity of the extragalactic gamma-ray background.
• Harrison, E. 2000, Cosmology: The Science of the Universe (second edition) (Cambridge: Cambridge University Press) A wide-ranging book,
placing the science of cosmology in its historical context, and discussing
its philosophical and religious implications.
• Islam, J. N. 2002, An Introduction to Mathematical Cosmology (second edition) (Cambridge: Cambridge University Press) A book which
emphasizes (as its name implies) the mathematical rather than the
observational aspects of cosmology.
• Liddle, A. 1999, An Introduction to Modern Cosmology (Chichester:
John Wiley & Sons) A clear and concise introductory work.
• Longair, Malcolm S. 1998, Galaxy Formation (Berlin: Springer-Verlag)
A well-written introduction to galaxy formation, approached from a
cosmological perspective.
• Narlikar, J. V. 2002, Introduction to Cosmology (third edition) (Cambridge: Cambridge University Press) Particularly useful for its section
on alternative (non-Friedmann) cosmologies.
• Peacock, J. A. 1999, Cosmological Physics (Cambridge: Cambridge
University Press) A large, well-stuffed grabbag of cosmological topics. Contains, among other useful things, a long, detailed discussion of
galaxy formation and clustering.
• Rich, J. 2001, Fundamentals of Cosmology (Berlin: Springer-Verlag)
Aimed primarily at physicists; provides a self-contained introduction
288
ANNOTATED BIBLIOGRAPHY
to general relativity, telling you as much as you need to know for cosmological purposes.
• Rowan-Robinson, M. 1996, Cosmology (third edition) (Oxford: Oxford
University Press) Has a slightly greater emphasis on astronomical observations than most cosmology texts.
• van den Bergh, S. 2000, The Galaxies of the Local Group (Cambridge:
Cambridge University Press) Cosmology begins at home: contains information about the distance scale within the Local Group, and its
dark matter content.
Advanced
• Kolb, E. W., and Turner, M. 1990, The Early Universe (Redwood City,
CA: Addison-Wesley) A text which helped to define the field currently
known as “particle astrophysics”.
• Liddle, A. R., and Lyth, D. H. 2000, Cosmological Inflation and LargeScale Structure (Cambridge: Cambridge University Press) Gives an
in-depth treatment of inflation, and how it gives rise to structure in
the universe.
• Peebles, P. J. E. 1993, Principles of Physical Cosmology (Princeton:
Princeton University Press) A classic comprehensive book.
Keeping Up to Date
Popular astronomy magazines such as Sky and Telescope and Astronomy provide non-technical news updates on advances in cosmology.
Scientific American, from time to time, includes more in-depth articles on cosmological topics. The Annual Review of Astronomy and
Astrophysics regularly provides reviews, on a more technical level, of
cosmological topics.
Index
baryons, 23
energy density, 179, 228
number density, 180, 228
bear
polar, 132, 152–153
teddy, 35, 47
Benchmark Model, 84, 85, 102–103,
118–123, 148, 195, 204, 235,
236
beryllium (Be), 209, 223
Big Bang, 6, 20–22
Big Bang Nucleosynthesis, 212
Big Bounce, 113
evidence against, 116, 150
Big Chill, 104, 106, 109, 113
Big Crunch, 105, 106, 111, 113
evidence against, 150
time of (tcrunch ), 106
blackbody radiation, 25
blueshift, 15
bolometric flux (f ), 132, 134
Euclidean, 133
Bondi, Hermann, 21
BOOMERANG, 196, 197
bottom-up scenario, 281
Abell 2218 (cluster of galaxies), 174
accelerating universe, 75, 150
acceleration equation, 67, 68, 128,
242, 268
with cosmological constant, 73
acoustic oscillations, 203
age of universe (t0 )
Benchmark Model, 112
empty universe, 87
flat universe
matter + lambda, 111
matter only, 94
radiation only, 95
single-component, 92
angular-diameter distance (dA ), 136,
138, 139, 196
maximum value, 140
related to luminosity distance,
138
astronomical unit (AU), 2, 131
axions, 276
baryon-antibaryon asymmetry, 229
in early universe, 230
baryon-to-photon ratio (η), 180, 190,
204, 220, 225, 230
smallness of, 229
baryonic Jeans mass (MJ ), 264
after decoupling, 265
before decoupling, 264
baryonic matter, 23
Cepheid variable stars, 141
period-luminosity relation, 142
cold dark matter, 276, 278, 281
power spectrum, 278–280
Coma cluster, 157, 164, 167–170
289
290
distance to, 168
mass of, 168, 170
comoving coordinates, 46
Copernican principle, see cosmological principle
Cosmic Background Explorer (COBE),
28, 182, 197
instruments aboard, 182
Cosmic Microwave Background, 28,
179
blackbody spectrum, 28, 182
compared to starlight, 82–83
cooling of, 30
dipole distortion, 183
discovery of, 180–181
energy density, 28, 179
number density, 28, 179
temperature fluctuations, 183,
196
amplitude, 185
and baryon density, 204
and flatness of universe, 204
correlation function, 197, 199,
200
origin of, 201–204
Cosmic Neutrino Background, 83,
175
energy density, 83
non-detection, 83
number density, 175
cosmic time (t), 46
cosmological constant (Λ), 68, 71–
75, 243
energy density, 73
pressure, 73
cosmological principle, 14
perfect, 21
INDEX
cosmological proper time, see cosmic time
critical density (εc ), 64
current value, 64
curvature
negative, 41
positive, 40
curvature constant (κ), 43, 46
dark energy, 71
dark halo, 162
dark matter, 27, 164
axions, 175
neutrinos, 175
nonbaryonic, 159, 164
primordial black holes, 175
DASI, 196
de Sitter universe, 98
deceleration parameter (q0 ), 127
Benchmark Model, 129
sign convention, 127
density fluctuations
lambda-dominated era, 269
matter-dominated era, 270
power spectrum, 274
radiation-dominated era, 269
density parameter (Ω), 65
baryons (Ωbary ), 159, 228
clusters of galaxies (Ωclus ), 170
cosmological constant (ΩΛ ), 84
galaxies (Ωgal ), 164
matter (Ωm ), 84, 155
radiation (Ωr ), 84
stars (Ω? ), 157
deuterium (D), 210
deuterium abundance, 227
determination of, 227
deuterium synthesis, 212, 216, 218
INDEX
compared to recombination, 219
temperature of, 222
deuterium-to-neutron ratio, 220
Dicke, Robert, 180
Digges, Thomas, 7, 10
distance modulus (m − M ), 148
Einstein radius (θE ), 172, 174
Einstein’s static universe, 74, 113
instability, 74
radius of curvature, 74
Einstein, Albert, 32–38, 57, 73, 74,
171
cosmological constant, 71
Einstein-de Sitter universe, 94
electron, 23
electron volt (eV), 3
empty universe, 86
expanding, 87
static, 86
energy density (ε), 57
additive, 80
baryons, 179
CMB, 179
flat universe
single-component, 92
matter, 80
radiation, 81
entropy, 66
equation of state, 69, 79
equivalence principle, 34
and photons, 38
and teddy bears, 35
expansion
adiabatic, 66
superluminal speed, 50
false vacuum, 251
291
driving inflation, 251
Fermat’s principle, 38
Fermi, Enrico, 24
first law of thermodynamics, 29, 66
flatness problem, 233–236
resolved by inflation, 244–245
fluid equation, 67, 68, 79
Ford, Kent, 162
Fourier transform, 273
fractional ionization (X), 187, 192
free streaming, 277
freezeout, 216
Friedmann equation, 58, 68, 79, 234
during inflation, 242
general relativistic, 61
Newtonian, 60
with cosmological constant, 73
Friedmann, Alexander, 58
fundamental force, 239
electromagnetic, 32
electroweak, 239
Grand Unified Force, 239
gravitational, 32, 33
strong nuclear, 32
weak nuclear, 32, 215
Galaxy (Milky Way), 2, 11, 160
luminosity of, 3
mass of, 163
mass-to-light ratio, 163
Galilei, Galileo, 34
Gamow, George, 181
Gaussian field, 274
geodesic, 38
geometry
Euclidean, 33
non-Euclidean, 55
gigayear (Gyr), 3
292
Glashow, Sheldon, 239
Gold, Thomas, 21
Grand Unified Theory (GUT), 238
phase transition, 240
gravitational instability, 258–261
gravitational lens, 171
cluster of galaxies, 174
MACHO, 171
gravitational potential energy, 165
gravity, Newton vs. Einstein, 38
Guth, Alan, 242
Harrison-Zel’dovich spectrum, 275
helium (He), 209, 223
helium fraction (Y ), 212, 225
primordial, 217–218
High-z Supernova Search Team, 146
Hilbert, David, 42
Hipparcos satellite, 143
homogeneity, 11, 13
horizon distance (dhor ), 10, 20, 93,
245
at last scattering, 237
Benchmark Model, 120
flat universe
matter only, 94
radiation only, 95
single-component, 93
horizon problem, 233, 234, 236–238
and isotropy of CMB, 238
resolved by inflation, 245–246
Hot Big Bang, 6, 30, 233
and production of CMB, 185
problems with, 233
hot dark matter, 276–277
power spectrum, 278
Hoyle, Fred, 21
INDEX
Hubble constant (H0 ), 15, 17, 18,
63, 127
Hubble distance (c/H), 20, 50
at time of last scattering, 201
Hubble friction, 248, 252, 268
Hubble parameter (H), 63
Hubble time (H0−1 ), 19, 75
relation to age of universe, 20,
92
Hubble’s Law, 15
consequence of expansion, 18
Hubble, Edwin, 15, 129, 141
Hydra-Centaurus supercluster, 183
hydrogen (H), 209
hydrostatic equilibrium, 169, 261
inflation, 242–253
and density perturbations, 253
and flatness problem, 244–245
and horizon problem, 245–246
and monopole problem, 246–247
inflaton field, 247, 252
ionization energy (Q), 187
isotropy, 11–13
Jeans length (λJ ), 262
radiation, 264
Jeans, James, 262
Keplerian rotation, 160
kinetic energy, 165
lambda, see cosmological constant
Langley, Samuel, 132
Large Magellanic Cloud, 142, 143,
172
distance to, 143
large scale structure, 255
last scattering, 186
INDEX
last scattering surface, 186
angular-diameter distance to, 196,
197, 238
thickness of, 195
Leavitt, Henrietta, 142
Lemaı̂tre universe, 113
lepton, 23
lithium (Li), 209, 223, 225
Lobachevski, Nikolai, 55
loitering universe, 113
evidence against, 116
lookback time, 120
Benchmark Model, 122
luminosity density, 20, 156
luminosity distance (dL ), 132, 136
flat universe, 135
related to angular-diameter distance, 138
M31 (Andromeda galaxy), 2, 11,
15, 160–162
distance to, 143
MACHO, 171–173
magnetic monopole, 241
energy density, 241
number density, 241
magnitude, 146
absolute, 147
apparent, 147
mass
gravitational (mg ), 33
inertial (mi ), 34
mass-to-light ratio, 156
cluster of galaxies, 170
Galaxy, 163
stars, 156
matter-dominated universe
negative curvature, 105
293
positive curvature, 104–105
ultimate fate of, 108
matter-lambda equality, 84, 111, 112
Benchmark Model, 118
Matthew Effect, 258
MAXIMA, 196, 205
Maxwell, James Clerk, 239
Maxwell-Boltzmann equation, 190,
214
mean free path, 188
megaparsec (Mpc), 2
metric, 43
curved space, 43, 44
flat space, 43
homogeneous & isotropic, 44
Minkowski, 45
Robertson-Walker, 46
Milne universe, 87
moment of inertia, 166
monopole problem, 233, 234, 238–
242
resolved by inflation, 246–247
neutrino, 24, 175
flavors, 24
massive, 25
oscillation, 25, 176
neutrinos, 277
neutron, 23, 213
decay, 23, 213, 222
neutron-to-proton ratio, 214, 215
after freezeout, 216
Newton, Isaac, 32–38
nuclear binding energy (B), 210,
211
of deuterium, 210, 218
of helium, 210
nucleons, 209
294
null geodesic, 45
in Minkowski space, 45
Robertson-Walker, 51
Olbers’ Paradox, 7–11, 20–21, 179
Olbers, Heinrich, 7
optical depth (τ ), 194
parallax distance, 132
parsec (pc), 2
particle horizon distance, see horizon distance
peculiar motion, 61
Penzias, Arno, 28, 180
perfect gas law, 69
perturbation theory, 259
phase transition, 239
loss of symmetry, 239
photodissociation, 25, 218
photoionization, 25, 187
photon, 25
photon decoupling, 185, 189, 193
photon decoupling time (tdec ), 194,
195
photon-baryon fluid, 195, 203, 264
Planck energy (EP ), 4
Planck length (`P ), 3
Planck mass (MP ), 4
Planck temperature (TP ), 4
Planck time (tP ), 4, 96
Poe, Edgar Allen, 10
Poincaré, Henri, 164
Poisson distribution, 275
Poisson’s equation, 57, 72, 202
with cosmological constant, 73
power spectrum, 274, 279
scale invariant, 274
pressure (P ), 57
INDEX
additive, 80
negative, 68
primordial black holes, 276
proper distance
Robertson-Walker metric, 49
proper distance (dp ), 47
at time of emission, 91
at time of observation, 89
flat universe
lambda only, 98
matter only, 94
radiation only, 95
single-component, 93
time of observation, 130
proton, 23, 213
Pythagorean theorem, 40
quantum gravity, 97
radiation-matter equality, 85, 117
Benchmark Model, 118
radiative recombination, 187
radius of curvature (R), 43, 46
recombination, 185
compared to deuterium synthesis, 219
recombination temperature
crude approximation, 190
refined calculation, 192
recombination time (trec ), 192, 195
redshift (z), 15
related to scale factor, 50–52
related to time, 86
redshift survey, 255
reheating, 252
Robertson, Howard, 45
Robertson-Walker metric, 46
Rubin, Vera, 162
295
INDEX
Sachs-Wolfe effect, 202
Saha equation, 191, 219
nucleosynthetic analog, 220
Sakharov, Andrei, 230
Salam, Abdus, 239
Sandage, Allan, 128
scale factor (a), 46
empty universe, 87
flat universe
lambda only, 98
matter + lambda, 109, 111
matter only, 94
radiation + matter, 117
radiation only, 95
single-component, 91
negative curvature
matter only, 108
positive curvature
matter only, 106
Taylor expansion, 126, 128
Slipher, Vesto, 15, 162
Small Magellanic Cloud, 142
solar luminosity (L¯ ), 3, 156
solar mass (M¯ ), 2
sound speed (cs ), 70, 262
baryonic gas, 265
photon gas, 265
Space Interferometry Mission (SIM),
143
spherical harmonics, 197
standard candle, 132
determining H0 , 141
standard yardstick, 136
angular resolution, 140
statistical equilibrium, 190
statistical weight (g), 190
Steady State, 21–22
supercluster, 13, 257
Supernova Cosmology Project, 146
supersymmetry, 176
Theory of Everything (TOE), 239
Thomson cross-section (σe ), 188, 215
Thomson scattering, 188
time of last scattering (tls ), 194, 195
Hubble distance, 201
top-down scenario, 278
topological defect, 240
cosmic string, 241
domain wall, 241
magnetic monopole, 241
triangle
in curved space, 40, 42
in flat space, 39
tritium (3 He), 222
Turner, Michael, 71
type Ia supernova, 145
luminosity, 145, 146
vacuum energy density, 75, 76
vs. Planck energy density, 76
Virgo cluster, 2, 143, 183
distance to, 143
Virgocentric flow, 144
virial theorem, 167
visible universe, 93
void, 13, 257
Walker, Arthur, 45
Weinberg, Steven, 212, 239
Wheeler, John, 38
Wilson, Robert, 28, 180
WIMP, 27, 177, 266, 276
year (yr), 3
Zwicky, Fritz, 164